PURPOSE: To ensure the accurate opening and closing operation of the pressure reduction valve, of an internal-combustion engine, by a mechanism in which the cylindrical rotor which is interlocked with the crankshaft, is provided with a moving control unit which is disengaged through centrifugal force and with a helical groove made on the support shaft which is capable of moving in the axial direction within the cylindrical rotor and is connected with the pressure reduction valve.

CONSTITUTION: The cylindrical rotor 34 rotates in interlocking with the crankshaft 22. The pin 36b on the moving control unit 36, when the designated rotating speed of the cylindrical rotor 34 is surpassed, is disengaged by the action of the centrifugal weight 36a from the helical groove 35a on the support shaft 35. As a result, the support shaft 35 is positioned outward through the torsion spring force applied thereto via the wire 39, closing the pressure reducing valve 30a. With the engine brought to a stop under this condition, the rotating speed of the cylindrical rotor 34 decreases corresponding to that of the crankshaft 22 to make the pin 36b engage with the helical groove 35 before the cylindrical rotor 34 stops. The resultant screw action works to pull the wire 39, rotating counterclockwise the activation lever 33 to open the pressure reducing valve 30a.