PURPOSE:To improve the vibration resistance performance by providing a buffer member to at least any face of counter face with a pressing member and a part of housing opposed to the pressing member when the support member supporting freely turnably the pressing member acting like disclamping is in the disclamping position. CONSTITUTION:The pressing member 52 has a magnet 52a formed in a ring form and applies disclamping with the magnetic force of the magnet. Moreover, the pressing member 52 and the support member 51 supporting freely turnably it are made of a magnetic substance such as a steel plate and when the support member 51 is in the disclamping position, the pressing member 52 is adsorbed to the support member 51 by the magnetic force of the magnet 52a. Further, a buffer member 52b made of felt is adhered to the upper face of the pressing member 52 so as to prevent the upper face of the pressing member 52 from being contacted directly with the housing 2 because of vibration of the support member when the support member 51 is in the disclamping position.
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