Just try re-potting it into a larger pot. It looks as if it could well be pot-bound.

And remember to open up the root ball when you do repot. To be honest Acers can only survive, and I do mean survive, for a few years in a pot - the really need to be in the ground.

Thanks for the comments. Best to repot now or leave until spring?

Repot now and keep well watered until November time.

That was my first thought too - root girdling. When you repot, look for encircling roots. Trim those off as Moon growe suggested. Bring up the soil level closer to the rim to give the roots more room.

Only just saw your post:
Your tree is a Japanese Maple (Acer Japonica, ssp. autopupereum) one of many varieties.
The problem seems to be that the tree is potbound and in dire need of repotting.
Don't worry, your tree can be saved and will thrive again in the same pot. This pot is ample big enough for this tree and I would estimate this pot is big enough for the next 20 years, no problem!
OK, now lets have some fun ^_-
Preparing the growing medium:
Go to the local pet store or buy online... non-clumping clay type cat litter. Yes you read that right, cat litter. You'll also need a piece of mesh (plastic or nylon) large enough to cover the pots drainage hole by a couple of inches and with small enough mesh to stop the cat litter from falling through.
Repotting:
Remove the tree from the pot and remove as much soil as you can, try not to damage too many roots. I use a BBQ skewer as much as I can then I wash off the remaining soil in a tub or use a hose.
Next try to untangle the roots as best you can.
Now (drum roll) cut off all the roots that are longer than 12".
For now leave the tree standing in the water keeping the roots wet.
Wash out your now empty pot, place the mesh over the hole and fill the pot with the cat litter to about 4" from the top. Using either a watering can or a hose gently pour on water ALL over the cat litter, you'll hear it hissing as the litter soaks up the water (I use Sanicat Pink non-clumping cat litter, it is light pink in colour when dry and turns a terracotta colour when wet. it also has a pleasant perfumed smell which is harmless to your tree) excess water will FLOW not seep through the drainage hole. this first flush of water will be pinkish in colour, it's the fine dust your washing away. when the water runs clear, stop.
In another container wash enough cat litter to top off the pot to 1 1/2" below the level of the rim of the pot.
place the tree into position in the pot tease the roots out so that they radiate horizontally out equally around the tree base, holding the tree in place gently cover the roots with the washed cat litter. Once done, still holding the tree use the BBQ skewer to poke the litter between the roots so that no air pockets remain. then spread the remaining cat litter over the roots taking care that the trees trunk is only covered to just the top of the highest root on the trunk.
Now place three or four hefty stones around the trunk to support the tree.
Place the pot in a shaded out of the wind place. Check it regularly to ensure the wind or some animal hasn't knocked it over.
Go into the house, pour yourself a drink, pat yourself on the back knowing thats a job well done and wait for spring ^_^
Do not feed until one month after bud break in the spring.
Welcome to the world of Bonsai. Bonsai literally means, Tree in a pot. Having said that, true bonsai is a Living Art
Form with lots of aspects to it, both physical and spiritual.
Like most trees in pots they will benefit from root pruning. Your aim is to have a rootball of about 12" after next pruning. Prune every year for best results.
To check if your tree needs watering, push a BBQ skewer into the cat litter, leave it 5 mins then take it out. If its damp or wet don't water. If it's dry, water.
I've read that the dust that is washed in the first wash if it can be saved and settled and dried is great for getting rid of spider mites.

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NOW BACK TO REALITY.
Benefits of using Cat litter.
*Cleaner. *Cannot be overwatered. *Allows air to the roots, essential to healthy root growth. *Inert, chemical composition won't change during any deteriation. Won't raise or lower your water PH. *Lighter weight than soil or composts. *Looks better. *Will not compact over time stopping air reaching the roots. *Allows free drainage keeping the roots free of standing in excess water. *it's easier to keep weed free. *Re-usable. *Relatively inexpensive. *Cats won't use it as a toilet if it's wet or damp. *Good for ALL types of plants, trees, shrubs, cacti, etc. *Cuttings "take" excellently in cat litter.

The downside of using cat litter.
Hmmm... thinking hard...oh yes. *You'll have to feed the tree more often, maybe as much as once a week or once every 10 dayzish. *There might be more, but honestly, i can't think of any.
Well, I've babbled and rambled on enough. I hope this is useful.