A fuel supply device is insertable into a fuel tank through an opening formed in an upper wall of the fuel tank in an insertion direction. The fuel supply device includes a flange unit and a pump unit coupled to the flange unit. The pump unit includes a guide member extending in the insertion direction. The guide member is configured to abut on an opening periphery so as to guide the pump unit when the pump unit is inserted into the fuel tank. A guide surface of the guide member includes a concave surface.