PURPOSE: To inexpensively and simply bond pipes by irradiating the pipes with a microwave using a cylindrical core having a specific structure.

CONSTITUTION: Pipes l composed of a thermoplastic resin are connected by irradiating the pipes with a microwave using a cylindrical core 2 provided in contact with the outer peripheral surfaces of the pipes in the connection parts of the pipes. The cylindrical core 2 has an intermediate layer 3 containing needle titanium oxide and composed of the same material quality as the pipes 1 on the inner peripheral side thereof and has at least one connection part 4. Needle titanium oxide is added to the thermoplastic resin forming the intermediate layer of the cylindrical core 2 by well mixing needle titanium oxide with a polymer soln. by a ball mill or a homogenizer.