A movable mold includes a movable mold main body and a gear piece attached to the movable mold main body so as to relatively movable, and having a cavity formation space for forming a cavity. In the gear piece, a piece-side coolant flow path includes an annular flow path surrounding the cavity formation space, a piece-side supply flow path introducing coolant into the annular flow path, and a piece-side discharge flow path introducing the coolant from the annular flow path. The piece-side coolant flow path is connected to a movable mold main body-side supply flow path of the movable mold main body and a movable mold main body-side discharge flow path of the movable mold main body and the coolant flows therein under negative pressure generated by a coolant circulation device, so the inside of the cavity can be cooled effectively.