A fuel supply device supplies fuel from a fuel tank to an injector by driving a diaphragm pump by use of pressure variation within a crankcase. The diaphragm pump includes: a pump casing; a diaphragm partitioning the interior of the pump casing into a negative pressure chamber and a pump chamber; and a plunger connected to the diaphragm and reciprocatively movable to and from the pump chamber. A pressure receiving area over which the plunger receives, at its end surface in a plunger advancing/retracting direction, pressure from the pump chamber is set smaller than a pressure receiving area over which the diaphragm receives pressure from the negative pressure chamber in the advancing/retracting direction.