It looks like an early blight. The best way to combat blight is prevention. Keep the bottom leaves from contact with the soil, water evenly and at the base of the plant rather than overhead watering. I would remove the affected leaves and put them in the trash rather than a compost pile.
Good sanitation is key. At the end of the season remove all vegetation from your growing area. A mulch program to reduce leaf contact with soil is essential since most leaf disease is soil borne. Strong, healthy plants combat diseases. Good airflow at the base of tomatoes will resist disease as well. Good basic growing practices are your best bet.
