The invention relates a photodetector, a liquid crystal display device, and a light-emitting device. One embodiment of the present invention includes a first light-blocking layer and a second light-blocking layer which are over a light-transmitting substrate, a first photodiode over the first light-blocking layer, a second photodiode over the second light-blocking layer, a first color filter covering the first photodiode, a second color filter covering the second photodiode, and a third light-blocking layer formed using the first color filter and the second color filter and disposed between the first photodiode and the second photodiode.