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) a nitrogen-containing polyphosphate compound, and (E) a curing accelerator. Also provided are an adhesive sheet having a layer including the above composition, and a protective layer for covering the layer including the composition; a coverlay film having an electrically insulating film, and a layer including the above composition provided on top of the film; a flexible copper-clad laminate having an electrically insulating film, a layer including the above composition provided on top of the film, and copper foil; and a method of bonding two substrates, including the steps of sandwiching the above adhesive sheet between two substrates, and curing the adhesive sheet. 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 anti-migration properties. The composition can be used for producing an adhesive sheet, a coverlay film, and a flexible copper-clad laminate. The adhesive sheet can be used for a method of bonding two substrates.