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.