Abstract: The present invention relates to a photosensitive thermosetting resin, which has more than two of ethylenically double bonds and carboxyl functional groups in one molecule thereof. The invention also relates to a solder resist ink composition, including (A) the above photosensitive thermosetting resin, (B) a photo-polymerization initiator, and (C) a thermo-hardener. The solder resist ink composition is suitably applied to a printed circuit board, and has excellent photosensitivity, solder resistance, electrical corrosion resistance, insulating properties and adhesion to copper foil.