A fuel-outflow check valve includes: a casing, at least one portion of which is located in a fuel tank, the casing having a ventilation hole via which the interior of the casing and the interior of the fuel tank communicate with each other and an upper wall portion protruding inward from the inner peripheral face of the casing; a float that is disposed in the interior of the casing and is vertically movable; and a valve element that is provided on the float and can contact a valve seat formed at the casing.