In manufacturing a resin mold assembly, a first weld region of a light transmitting resin member (22) and a second weld region of a light absorbing resin member (21) are disposed facing each other, the first weld region of the light transmitting resin member (22) and the second weld region of the light absorbing resin member (21) are brought into pressed state in mutually facing direction, and a plurality of laser beams (12s) is scanned over a whole area of the second weld region, and repetitively radiated on the second weld region to heat and melt a whole of the first and second weld regions simultaneously, thereby welding the light transmitting resin member (22) and the light absorbing resin member (21).