This detection device detects an attack in an on-vehicle network that includes a bus in which a frame including identification information that allows recognition of at least one of a transmission source and a destination is transmitted. In the bus, a plurality of the frames including pieces of the identification information different from each other are transmitted. The detection device includes: a monitoring unit configured to monitor a communication error in the bus; an aggregation unit configured to aggregate a communication error occurrence state regarding each piece of the identification information on the basis of a monitoring result by the monitoring unit; and a detection unit configured to detect the attack on the basis of an aggregation result by the aggregation unit.