An arithmetic device includes: an input unit to which information pertaining to a detection target that is a target recognized by a sensor is input from the sensor; a processing-unit allocation unit that is configured to allocate a plurality of the targets to any one of a plurality of groups; an association unit that is configured to retrieve a second one of the targets to be associated with a first one of the targets from a part of the plurality of groups included in the plurality of groups; and a state fusion unit that is configured to fuse the first target and the second target that have been associated with each other by the association unit to produce a tracking target that is a target being tracked. The first target is any one of the detection target and the tracking target. The second target is any one of the detection target and the tracking target.