To facilitate re-assembling of a throttle valve in an accelerator operating a piston of an acceleration pump by a cam formed on the throttle valve.

A cam groove 33 is formed on the throttle valve 4 so as to open the bottom surface. An engagement projection 29 projected on the piston 27 of the acceleration pump 25 is engaged into the cam groove 33. The piston 27 is linearly reciprocated according only to the cam groove 33. When the throttle valve is drawn out, the piston 27 stops at a position as before drawing out. When the throttle valve 4 is re-assembled, the engagement projection 29 is engaged into the cam groove as it is. Thereby, it is not needed that the acceleration pump is disassembled, or that the dimensional relation is strictly set in order to maintain stopping of the piston 27.