A mounting unit that can be worn easily and in which size adjustment and lock release can be operated with a natural hand position is disclosed. The mounting unit includes a front frame and a rear frame. The rear frame being connected to the front frame can move in a back and forth direction with respect to the front frame. And the unit includes knob members which are provided on both the right and left portions of the rear frame, for operation of backward movement of the rear frame. Furthermore, the unit includes lock mechanisms which prevent the backward movement of the rear frame with lock release buttons which allow release of the lock of the lock mechanisms. The knob members are arranged under both frames. Each of the lock release buttons is arranged at a portion on each of the knob members where the user pushes with his/her finger when a user will move the rear frame backward with respect to the front frame.