There is provided a flame retardant composition for a solder resist which is halogen-free and has a high level of flame retardance and flexibility, while exhibiting excellent heat resistance, moisture resistance and high-temperature reliability. The flame retardant composition for a solder resist according to the invention comprises (A) an alkali-soluble resin comprising either or both (A1) a carboxyl group-containing epoxy (meth)acrylate or (A2) a carboxyl group-containing urethane (meth)acrylate obtained by reacting an epoxy resin with two or more epoxy groups in the molecule (a), an unsaturated group-containing monocarboxylic acid (b) and a polybasic acid anhydride (c), (B) a compound having an ethylenic unsaturated group in the molecule, (C) a photopolymerization initiator, (D) a phosphorus-containing epoxy resin having a specific structure and (E) a hydrated metal compound. The composition of the invention can be suitably used as a solder resist or cover lay film for an FPC.