A recess 41 having an inner peripheral surface 55 formed by a first inclined surface having a diameter reduced from the outer surface of the frame 34 toward the bottom 56 and a push-type operation button 42 attached to the recess 41 are formed as a first The operation switch 35 is configured to include a cylindrical portion 60 in which the outer peripheral surface 61 is configured by a second inclined surface facing the inner peripheral surface 55 of the concave portion 41 with a larger inclination than the inclined surface.フレーム３４の外表面から底部５６に向かって縮径する第１の斜面によって内周面５５が構成された凹部４１と、凹部４１に装着される押圧式の操作ボタン４２に形成され、第１の斜面よりも大きい傾きを有して凹部４１の内周面５５に対向する第２の斜面によって外周面６１が構成された筒状部６０と、を有して操作スイッチ３５を構成する。
