Hello Kathy,

This could be Phyllosticta leaf spot disease.  I'm not sure if you're noticing other signs like spots with light colored centers.  Here is a page showing several maple leaf issues but I'm zeroing in on Phyllosticta.  Unfortunately, the link to further information does not include anything more about Phyllosticta. 

It's not a definitive diagnosis but you can get one by sending in a live sample, like the one you photographed, to the U of MN Plant Disease Clinic.  Here is a link to their page where you'll learn about their services, prices, and how to submit a sample.

Here is more information from another resource about Phyllosticta that basically says that it shouldn't affect the health of your tree but there are hygienic measures you can take to control it.  I recommend collecting and disposing all leaves that fall off now and into the fall.  And by disposing I mean burning or burying the leaves.  If they're allowed to overwinter on the ground, they could reinfect the tree next spring when it rains. 

I hope this helps.