PURPOSE: To facilitate the bonding of a polyolefin film with a high effect of adhesiveness improvement without causing lowering in mechanical strength, by sticking a polyolefin film through a layer contg. a polyolefin and an org. peroxide on the surface of an adherend and carrying out a crosslinking reaction.

CONSTITUTION: A polyolefin film or sheet mainly composed of a polyolefin and contg. a component (a) capable of co-crosslinking with the polyolefin and a tert. amine or a metal salt (b) capable of undergoing a redox reaction with an org. peroxide, is sticked through a compsn. layer contg. an org. peroxide and a component (c) capable of co-crosslinking with component (a) and contg. a polar group, on the surface of an adherend, and a crosslinking reaction is carried out to bond them to each other. In the crosslinking reaction, the polyolefin of the film or sheet, component (a) and component (c) in the compsn. layer are co- crosslinked and the film or sheet is bonded to the compsn. layer by crosslinking. Since component (c) contains a polar group, the bonding of the film or sheet to the adherend is firmed as crosslinking proceeds.