A detection device includes: a sensor in which detection elements each including a photoelectric conversion element are arranged in a detection region; a drive circuit configured to supply drive signals to the detection elements; and a detection circuit configured to process a detection signal output from each detection element. Each detection element includes: a source follower transistor configured to output a signal corresponding to an electric charge generated in the photoelectric conversion element; and a read transistor configured to read an output signal of the source follower transistor and output the detection signal. A potential obtained by superimposing a threshold voltage of the source follower transistor and a voltage drop caused by on-resistance of the read transistor on a reset potential serving as a reference value for a voltage variation amount of the detection signal is set as an initial value of a potential applied to the photoelectric conversion element.