The invention relates to an evaporated fuel treating device that includes: a canister configured to adsorb evaporated fuel generated in a fuel tank through a vapor passage; a blocking valve configuredto close and open the vapor passage, the blocking valve having a stepping motor as a drive source; and a controller configured to operate the blocking valve by controlling electric power that is supplied from a predetermined power supply to the blocking valve. The controller is configured, when there is a request to drive the blocking valve, to operate the blocking valve by controlling the electric power such that when a voltage that is supplied from the power supply to the blocking valve is less than a predetermined value, a driving period which is an interval between pulses that are supplied to the stepping motor becomes long compared to when the voltage is equal to or higher than the predetermined value.