Disclosed is a thermosetting resin composition essentially containing (A) a radical (co)polymer obtained by (co)polymerizing an ethylenically unsaturated monomer (a) having one or more alicyclic epoxy groups in one molecule and another ethylenically unsaturated monomer (b) which is used if necessary and copolymerizable with the monomer (a) using a non-nitrile azo compound as a polymerization initiator; (B) at least one substance selected from polybasic acid anhydrides and polybasic acids; and (C) a curing accelerator. Also disclosed are a sealing material for optical devices and a cured product. The thermosetting resin composition provides a cured product which is high in transparency and Tg while having good light resistance and good crack resistance. The thermosetting resin composition also provides a sealing material for optical devices which is high in transparency and Tg while having good light resistance and good crack resistance.