An automatic injecting type paste form heat-blowing injection composition comprising a partially crosslinked rubber, an unvulcanized rubber, a crosslinking agent, a plasticizer, a thermoplastic resin, an epoxy resin and a latent curing agent thereof, and a blowing agent. This composition can be used in a injecting and soundproofing method comprising forming an insulation wall by heating and foaming of a injection composition in a closed section of an automobile body part having a closed section frame which is produced by press molding in a body welding step of an automobile manufacturing line, and it has a good anti-stringy property and suffers form less sagging during heating and foaming.