are provided at opposite sides of the film heat can be generated from the film 15 substantially uniformly over the entire area thereof without resulting in irregularities in temperature the conductive film 15 can be seen through and therefore will not obstruct the view further because the conductive film 15 is colored and transparent the eyes of the wearer can be protected from ultraviolet rays infrared rays etc by reflection when the present device is used on a motorcycle 3 or like vehicle running at a high speed the shield panel 6 will be cooled with air over its surface such that unless a great current is passed through the conductive film 15 the amount of heat released is small with the likelihood that the shield panel 6 will not be heated to a temperature at which freezing can be prevented effectively however if the device is set fixedly to a temperature based on the air cooled state the shield panel 6 will be heated to an excessively high temperature when the vehicle stops during running according to the present invention therefore the temperature sensor 22 is adapted to detect the temperature of the heat generating plate 16 for the temperature controller 23 to control the current to be passed through the conductive film 15 when the temperature of the heat generating plate 16 varies with.