PURPOSE: To obtain a primer sheet capable of enhancing the bonding strength of a polyethylene sheet and a steel pipe, by a method wherein a thermosetting resin and a thermoplastic resin having a melt viscosity higher than that at the time of setting of the thermosetting resin are mixed with each other in a weight ratio of 9/1W1/9.

CONSTITUTION: A thermosetting resifn and a thermoplastic resin having a melt viscosity higher than that at the time of setting of the thermosetting resin are mixed with each other in a weight ratio of 9/1W1/9. This mixing is conducted under heating or in a dry system. Thereafter, the mixture is extruded to a predetermined thickness to produce the primer sheet. The sheet is wound around a steel pipe, a polyethylene sheet is placed thereon and is adhered thereto by heating. Accordingly, unlike the case of using a conventional primer treating liquid, the possibility of polluting the working area is completely eliminated, an adhering operation is facilitated, and a high bonding strength can be obtained.