A temperature sensor is provided backward an imaging device. A temperature of the imaging device is always detected when a through image is displayed and result of measurement is output to an MPU. The MPU compares a temperature t input from the temperature sensor with a predetermined threshold value tc. When the temperature t of the imaging device becomes a value equal or higher than the predetermined threshold value tc, a warning display continues for a predetermined time, for example 15 seconds and then the through image on the backside liquid crystal display is stopped.