A monitoring lens device includes a focus ring that is provided in an outer circumferential portion of a fixed cylinder and a zoom ring that is provided in an outer circumferential portion of the focus ring. A screw hole that is engaged with a shaft portion of a lock knob is formed in the focus ring, and an opening portion (long groove) through which the lock knob passes is formed in the zoom ring. A spacer is attached to the lock knob and a convex portion of the spacer is fitted into the opening portion.