A refueling portion structure of a fuel tank includes: a refueling pipe that has a refueling port into which a refueling nozzle is to be inserted, and that communicates with a fuel tank; an opening/closing valve that is provided on a flow path within the refueling pipe and closes the flow path, and that, when pushed by the refueling nozzle, opens the flow path; and a valve opening/closing device that, in a case in which a tank internal pressure of the fuel tank exceeds a set value that is set in advance, restricts or inhibits opening operation of the opening/closing valve, and, in a case in which the tank internal pressure is less than or equal to the set value, permits the opening operation of the opening/closing valve.