A flexing strip (41) is formed on a housing (20). An interference part (63) is formed on a fit-on detector (50). A distinguishing part (46) having a color different from the fit-on detector (50) is formed on the flexing strip (41). The flexing strip (41) interferes with the interference part (63) and deforms while moving the fit-on detector (50) from a wait position to a detection position. The flexing strip (41) is released from an interference state with the interference part (63) and elastically restores to its original state when the fit-on detector (50) has reached the detection position. A position of the distinguishing part (46) relative to the interference part (63) changes before and after a flexing of the flexing strip (41) so that the distinguishing part (46) is exposed and can be seen from a distinguishing position.