Abstract: A movable hook portion (13) provided in a unit side coupling surface (2a) of an accessory unit (2) includes a hook portion main body (14), which is formed with a movable claw (23a) and a pressed piece (22), and a straight spring (15), slidably accommodates the hook portion main body (14) in a main body accommodation concave portion (16) formed in the unit side coupling surface (2a), and accommodates the straight spring (15) in a spring accommodation concave portion (17a) formed in the unit side coupling surface (2a) so as to be orthogonal to a sliding direction of the hook portion main body (14), with a middle portion in a length direction of the spring being engaged with the hook portion main body (14).