A pasty heat-expandable filler composition of the automatic application type which comprises a partially crosslinked rubber, an unvulcanized rubber, a crosslinking agent, a plasticizer, a thermoplastic resin, an epoxy resin and a latent hardener therefor, and a blowing agent. The composition can be used in the filling/sound insulation technique in which a car body member which has been produced by press molding in a body step of a motor vehicle manufacturing line and which has a closed sectional structure is processed to fill the closed section with an insulation wall through thermal expansion. The composition is free from stringiness during application and can be prevented from sagging during thermal expansion.