Effective cooling channels have been installed in the injection mold during the process of milling-combined laser metal sintering. In the case of molding a flat piece with multiple ribs, it is difficult to insert cooling channels between the ribs by conventional machining. Because the heat remains between the ribs, the piece shrinks after the molding to form a warp on the flat surface. Using milling-combined laser metal sintering, oval holes were fabricated between the ribs and they were connected in a spiral loop. The temperature around the ribs became uniform and the warp on the flat surface of the molded specimen reduced compared with that molded in the mold made by conventional method using milling and electro-discharge machining. Simulation of the mold temperature and the warp by commercial software showed the same tendency on the change of the warp with the mold type and the cooling temperature conditions. Therefore, the accuracy of the molded specimen is improved by inserting cooling channels in the mold where heat accumulates using a milling-combined laser metal sintering and the effect can be evaluated by a simulation.