PURPOSE: To provide a weld line reinforcing method wherein sufficient strength can be obtained, the cost is advantageous, process is not so complicated, and to provide a mold for carrying out the method.

CONSTITUTION: A small space 22 for lowering a pressure of either a resin flow 18 or a resin flow 20 which is injected from one or two gates 12, 14 becoming a confluence in a cavity 16 is provided so as to communicate with the cavity 16 at a position nearer to the gate 12 than a position where a weld line is to be formed. After a full shot, a valve 22B at an inlet of the small space is artificially opened or automatically opened in response to the resin pressure. By opening and closing the gates, one resin pressure of the resin flow is lowered and other resin pressure is raised relatively, thereby forming a weld line having deep irregularity for improving the strength and appearance.