A liquid crystal display device 1 having a liquid crystal display panel and an illuminating means controlled according to the output of the light sensing component. The light sensing component is deployed at the periphery of the TFT substrate's display area DA and uses a thin film transistor as photosensor. A capacitor C is connected between source and drain electrodes SL, DL for such TFT photosensor. One of the capacitor's terminals is connected to a standard voltage source VS via a switch element SW, and the other to the common electrode 26. Voltage that is always lower than the voltage applied to the common electrode by an amount corresponding to a reverse bias voltage is applied to the gate electrode GL for the TFT photosensor. The capacitor's voltage a certain time after the switch element turns off is output.