which tends to advance it tends to be reversely rotated thus decelerating the pinion 5 further if an accelerating force acts on the pinion 5 the driving shaft 4 is retracted the sleeve shaft 19 has a cylinder 24 fitted thereon through a needle bearing 23 and the doughnut like air piston 3 is slidably installed in a doughnut like space defined by said cylinder 24 and piston housing 2 the air piston 3 is normally rearwardly pressed by a piston spring 26 through a sleeve 25 the sleeve 25 is integral with the outer race of a bearing 26 39 the inner race of said bearing 26 39 is integral with the driving shaft 4 thus the sleeve 25 and driving shaft 4 are integrally movable back and forth but the sleeve 25 is not rotated by the driving shaft 4 the details of main valve 6 sub valve 7 and start signal retention valve 104 are as follows in fig1 these valves 6 7 and 104 are mounted on the main valve 12 that is the main valve 6 is contained in a main valve cylinder 27 39 the sub valve 7 is contained in a valve housing 27 and the start signal retention valve 104 is contained in a start signal retention valve body 29 said cylinder and.