To provide a mount case which can adjust the distance between a magnet of drop-in isolator in a microwave circuit and the main body of case (gap length) without being affected by the height of drop-in isolator and can move a resonance frequency to the outside of the pass band of the drop-in isolator.

A drop-in isolator 3, for which a terminator for basic waves is provided at one part of film-form conductor 38 formed on a magnetic material substrate 31, is arranged inside the main body 1A of this mount case and between a magnet 6 of terminator and the main body 1A of case facing this magnet, a spacer 5 composed of a non-magnetic substance is provided so that a bias magnetic field impressed by this magnet 6 can be adjusted. In this case, a movable part for adjusting the distance (gap length) between the main body 1A of case and the magnet 6 is provided at the connecting part of spacer 5 to the main body 1A of case.