PURPOSE: To prevent a useless display at an unused time after a use of a device is ended by applying a voltage erasing display contents written on a liquid crystal panel based on the information of power source off between a scan line and a signal line.

CONSTITUTION: A circuit 4 detecting on/off of a power source switch 3 is assembled in a power source circuit part 2 of an electrooptical device provided with the liquid crystal panel 1, and the capacity of a smoothing capacitor 5 assembled in the output side of the power source circuit part 2 is enlarged beforehand. Then, the detection circuit 4 detects the off of the power source switch 3, and it outputs a control signal to a display controller 8 so that a driver LSI 6 for scan and a driver LSI 7 for data output an erase signal instantly. At this time, one side scan electrodes in matrix are selected successively, and though the erase signal is applied to every row, the smoothing capacitor 5 holds the source voltages of the driver LSI 6 for scan and the driver LSI 7 for data until the erase signal continues for one frame or above, and it erases the whole picture.