To prevent the generation of bedew and to avoid heat crack by additionally providing a planar heat generator on the rear side of a mirror and suppressing thermal stress, which is generated at the edge part of glass plate, less than specified by specifying the capacitance of the heat generator corresponding to the unit area of the mirror.

The rapid heating of the mirror is avoided and always or excepting for sleeping at least, the mirror is set at the temperature higher than the room temperature for several °C or less than 10°C for a long time. Besides, the planar heat generator is additionally provided on the rear face of the mirror and this planar heat generator is adopted with the design of capacitance less than 120W/m2 and more than 30W/m2. Otherwise, when adopting the planar heat generator of much higher output, it is enough only to previously set the capacitance less than 120W/m through an output control mechanism and in case of use, the planar heat generator is always electrified or the electrification is stopped only during sleeping by switch operation. As a result, the generation of bedew can be prevented, sharp reflected images can be always watched, thermal stress at the terminal edge part of the mirror can be suppressed less than 55kg/cm2 as well and the generation of heat crack can be practically eliminated.