A detection device is provided and includes insulating substrate; detection element including first photodiode, first thin film transistor, second thin film transistor, and third thin film transistor; dummy element including second photodiode and first thin film transistor; and first scan line, wherein first scan line is a gate of each of first thin film transistors, dummy element is located adjacent to detection element, and dummy element includes first photodiode and none of second thin film transistor and third thin film transistor.