A swirling wall structure-extends from a lower side toward an upper side in a sub-tank, and a fuel flow, which is outputted into an inside of the sub-tank from a flow outlet of a diffuser passage opened toward a lateral side, is swirled by the swirling wall structure. The swirling wall structure-includes a curved wall surface and a U-turn wall surface. The curved wall surface is curved about a longitudinal axis, which extends from the lower side toward the upper side in the sub-tank, to bend the fuel flow outputted from the flow outlet. The U-turn wall surface extends continuously from the curved wall surface to make a U-turn of the fuel flow, which is bent by the curved wall surface.