A method of casting a casting product having tubular flow passages includes: attaching a cast-in pipe insert, having outer and inner fixing members to which both ends of pipes are coupled, respectively, to a fixed mold; assembling a movable mold with the fixed mold; injecting a molten metal into a cavity defined inside the fixed and movable molds; ejecting a casting product from the assembled molds after injecting the molten metal; and removing the outer and inner fixing members from the casting product.