Abstract: A casting method for forming a fastening part on an inclined plane of a casting may include mounting a plug unit including a casting plug and a screw thread insert combined with the casting plug on a movable mold or a fixed mold; performing casting by moving and combining the movable mold with the fixed mold and injecting a molten metal into a cavity formed between the movable mold and the fixed mold; performing mold opening after the casting; and separating the casting plug from the screw thread insert, wherein when the plug unit is mounted on the mold, the screw thread insert is deployed in the cavity, and a mold plane on which the casting plug is mounted forms an inclined plane having an acute angle against a horizontal plane that is parallel to a movement direction of the movable mold.