According to an aspect, a detection device includes: a base member having a first principal surface; a detection area provided to the first principal surface; a sensor including a plurality of photodiodes; gate lines; signal lines; and a light emission device. The light emission device includes light sources. A lit area in which the light emission device is lit up is a portion of the detection area overlapping the light sources. A portion of the detection area not overlapping the lit area is an unlit area in which the light emission device is not lit up. Photodiodes that overlap the unlit area are selected from among the photodiodes, and amounts of light received by the selected photodiodes are read.