Provided is a flame retardant adhesive composition including (A) a halogen-free epoxy resin, (B) a thermoplastic resin and/or a synthetic rubber, (C) a curing agent, (D) an organophosphinate compound, (E) a curing accelerator, and (F) a nitrogen-containing organophosphate compound represented by a general formula (1) shown below: (wherein, R1 to R4 each represent a hydrogen atom or an alkyl group, and X represents a nitrogen-containing bivalent organic group) Also provided are an adhesive sheet having a layer comprising the above composition, and a protective layer for covering the layer comprising the composition; a coverlay film having an electrically insulating film, and a layer comprising the above composition provided on top of the film; and a flexible copper-clad laminate having an electrically insulating film, a layer comprising the above composition provided on top of the film, and a copper foil provided on top of the layer comprising the composition. Further provided are a process for producing the adhesive sheet, a process for producing the coverlay film, and a process for producing the flexible copper-clad laminate. The halogen-free adhesive composition yields a cured product, on curing, that exhibits excellent flame retardancy and electrical characteristics (anti-migration properties). The composition can be used for producing an adhesive sheet, a coverlay film, and a flexible copper-clad laminate.