PURPOSE: To permit visual checking of the presence or absence of the oscillation of microwaves with ease by installing a fluorescent tube in the electric effect by microwaves and transmitting and displaying the fluorescence of the fluorescent tube emitted by the field effect by means of an optical fiber to a display part.

CONSTITUTION: A fluorescent tube 38 is installed on the rear surface of a cover 15a of a fixing device and the tube 38 is disposed in the electric field slightly apart from the strong electric field of microwaves but in the field strength sufficient to emit fluorescence. Since this fluorescence is transmitted to an operating part 40 on the top surface of the body of an image former, the user is able to check the oscillation of microwaves by viewing this light.