It looks like Septoria Leaf Spot. It is a fungal disease (see below for a link to a Fact Sheet on Tomato issues).  Good sanitation will help.  Remove diseased leaves (though too many can weaken the plant, so use your best judgment), practice plant rotation next year as the fungus overwinters in the soil.  The fruit should remain unaffected, but at the end of the season, dispose of the plant, do not compost and remove all fallen leaves. 

Recognizing Tomato Problems: https://ask.extension.org/expert/questions/470217

Please let us know if this was helpful or if you have any further questions.

Thanks for contacting the Master Gardeners

Sandra and Judy