In-Vehicle Fuel Tank Device and Utility Vehicle

An in-vehicle fuel tank device includes an engine; a main tank including a top portion, a side portion, and a bottom portion; and a fuel pump configured to supply fuel to the engine. The bottom portion includes a first area and a second area, at least a portion of the first area having an inclined surface, at least a portion of the second area being depressed to serve as a sub tank configured to receive and store fuel having flown along the inclined surface. The fuel pump includes a suction filter inside the sub tank.