A reset knob 20 is mounted to a tip end of a shaft 16. A torsion spring 21 and a returning motion-preventing lock 22 are bent and accommodated in the reset knob 20. The returning motion-preventing lock 22 can rotate around a reset-knob fulcrum 20a of the reset knob 20 by a load of the torsion spring 21. Therefore, the returning motion-preventing lock 22 jumps out from inside of the reset knob 20 which is firmly connected together with the shaft 16 by a force of the torsion spring 21, thereby preventing the shaft 16 and the reset knob 20 from returning to an initial position. Further, since the returning motion-preventing lock 22 can easily be accommodated in the reset knob 20 manually, a reset operation for returning the shaft 16 to its original initial position can be carried out easily.