To form a discharge gap to protect a circuit on a small printed-circuit board to which commercial AC power supply is inputted.

An AC-DC converter to which commercial AC power supply is supplied is installed on a small printed-circuit board 18. A high frequency amplifier which operates based on electric power from the AC-DC converter is installed on a separate printed-circuit board 7 and is housed in a shield case 8. Connection ends 24a, 24b of routes 22a, 22b for the commercial AC power supply are installed at one of the fringe of the printed-circuit board 18, and the shield case 8 is provided opposed to this fringe with a prescribed gap G.