The present invention relates to a method of manufacturing a slag discharge door, which is provided separately from a molten-steel discharge opening in an electric furnace and opens and closes a slag discharge opening for discharging slag. The method of manufacturing the slag discharge door, which moves up and down on the slag discharge opening to open and close the slag discharge opening, includes the steps of: forming a door body by forging, forming a coolant passage, a coolant inlet, and a coolant outlet in the door body by drilling, and then finishing a passage that blocks a portion that is open toward an outer surface of the door body.