A back mirror 1 comprising an automatic glare-proof function of the present invention includes a battery E1, a daytime/nighttime deciding circuit 3 for deciding whether the current time is daytime or nighttime, an integrating circuit 4, an electronic switch FET1, a glare-proof/glare-non-proof deciding circuit 5 for deciding whether an EC mirror 2 is set to a glare-proof state or a glare-non-proof state, and a driving circuit 6 for applying a DC voltage to the EC mirror 2. The daytime/nighttime deciding circuit 3 is being constantly applied with a DC voltage from the battery E1. When the daytime/nighttime deciding circuit 3 has decided that the current time is daytime, the electronic switch FET1 is turned off, and when the daytime/nighttime deciding circuit 3 has decided that the current time is nighttime, the electronic switch FET1 is turned on. Therefore, the voltage is applied to the glare-proof/glare-non-proof deciding circuit 5 and the driving circuit 6 only when a decision has been made that the current time is nighttime. Thereby, the power consumption of the battery E1 can be decreased.