To provide a reactive flame retardant having all of various physical properties such as heat resistance, hydrolysis resistance, processability (solvent solubility) and adhesivity and photosensitivity, flame retardancy, softness, flexibility and sufficient mechanical strength and capable of sufficiently coping with the miniaturization and weight reduction of electronic component in electronic equipment, and to provide a photosensitive resin composition using the reactive flame retardant and a typical application thereof.

The reactive flame retardant in which a phosphazene compound having (a) phosphazene structure and containing (b) photosensitive group in a molecule is cross-linked by (c) an ester bond is used.