The present invention provides a liquid crystal display device capable of preventing the occurrence of dark currents in photodiodes. Thus, the liquid crystal display device includes a liquid crystal display panel 1 including an active matrix substrate and a backlight 13 for illuminating the liquid crystal display panel. The active matrix substrate 1 includes a photodiode 7 formed by a silicon film and a light shielding film 8 for shielding the photodiode 7 against illumination light from the backlight 13. The photodiode 7 and the light shielding film 8 are provided on a base substrate 5. The light shielding film 8 is formed by a semiconductor or an insulator. Preferably, the photodiode 7 is made of, for example, polycrystalline silicon or continuous grain silicon so as to have a characteristic that its sensitivity increases as the wavelength of light entering the photodiode becomes shorter. The light-shielding film 8 is formed by a silicon film, for example amorphous silicon, that reduces the transmittance of light entering the light shielding film as the wavelength of the light becomes shorter.