A fuel-pump-driving device (20) drives a plunger (31) of a fuel pump (30) to perform a reciprocating motion to supply fuel to an internal combustion engine. The fuel-pump-driving device (20) comprises a cam (27) that drives the plunger (31) according to the rotation of a fuel-pump-driving sprocket (26). The fuel-pump-driving sprocket (26) meshes with a chain (7A) which travels between a crank sprocket (3) and a valve-driving sprocket (5A) of the engine. Since the fuel-pump-driving sprocket (26) can be located in a position detached from the valve-driving sprocket (5A), the fuel-pump-driving device (20) has greater freedom of layout and the fuel pump (30) exhibits a better performance than in a case where the plunger (31) is driven by a cam that is fixed directly to the cam shaft (5) of the valve-driving sprocket (5A).