To conveniently judge quality of welded condition of a resin member consisting of a thermoplastic resin to be acceptable or not.

A pair of the resin members 11 consisting of the thermoplastic resin are prepared and notches 13 to disappear by an appropriate welding are formed on welding end parts 12 of the resin members 11 and a pair of the resin members 11 on which the notches 13 are formed are arranged so as to face each other and the welding end parts 12 of these resin members 11 are melted by heating and the heat-melted welding end parts 12 of the resin members 11 are brought into press contact with each other to weld together a pair of the resin members 11 and the quality of the welded condition is judged to be acceptable or not by the existence of the notches 13 after welding.