A liquid crystal display device 1 according to an embodiment of the present invention includes a liquid crystal display panel, a photodetector unit LS1 that is built into the liquid crystal display panel and has a TFT ambient light photosensor, an ambient light photosensor reader Re1 for reading a voltage due to photo-leakage from the TFT ambient light photosensor, and a control means 20 for controlling the photodetector unit and the ambient light photosensor reader, and a backlight 24, etc., according to an output from the ambient light photosensor reader. The photodetector unit has a switching element SW2 coupled between a source line coupled to the TFT ambient light photosensor's source electrode SL and a drain line coupled to the TFT ambient light photosensor's drain electrode DL. The control means 20 makes the source line and the drain line of the TFT ambient light photosensor be coupled to a common potential when changing a gate voltage of the TFT ambient light photosensor by keeping the switching element SW2 turned on. It is therefore possible to provide a liquid crystal display device that utilizes a thin film transistor as an ambient light photosensor and prevents degradation of an ambient light photosensor element due to a biased polarity voltage applied to the gate electrode.