A photosensitive resin obtained by reacting a reaction product (I) prepared from a biphenyl epoxy acrylate (a) of the formula (1) and a cyanate ester compound (b) with a polybasic acid anhydride (c), and a photosensitive resin and an epoxy compound, wherein each of R1 and R9 is a hydrogen atom or methyl, and n is an integer of 1 or more. The photosensitive resin and the photosensitive resin composition have excellent developability, has high heat resistance and particularly has excellent heat resistance and reliability on electric insulation under moisture absorption lasting for a long period of time.