An evaporated fuel processing device that includes a fuel tank; a vapor passage through which evaporated fuel generated from a fuel in the fuel tank flows; a closing valve configured to open and close the vapor passage; a concentration sensor configured to detect a concentration of the evaporated fuel in the vapor passage downstream of the closing valve; and a controller. When the closing valve moves toward an open side in the closed state, the controller may specify a valve-opening-start position of the closing valve based on a concentration detected by the concentration sensor, wherein the valve-opening-start position is a position where the closing valve transitions from the closed state to the opened state.