A protrusion on a first member made of synthetic resin wherein a laser beam is applied to a side surface of the protrusion in a state in which a top surface of the protrusion of the first member is abutted against a second member made of synthetic resin, so as to melt at least the entire top surface of the protrusion and melt a portion of the second member in contact with the protrusion by heat of the melted top surface of the protrusion, followed by solidification of the melted portions, whereby the first member and the second member are welded together.