Device for calculating bundle diameter of electrical wire bundle

A bundle diameter calculation device calculates the number of electrical wires belonging to an electrical wire bundle constituting a part of a wire harness of which each of the plurality of electrical wires is connected via relay points with connectors, the device including: a storage unit which stores electrical wire-connector connection information in which each of the plurality of electrical wires and the connectors are associated with each other, and bundle connection information which shows a state of connection with the connectors and relay elements by the unit of the electrical wire bundle of the wire harness; and a processing unit which, when the wire harness is specified, checks the electrical wire-connector connection information and the bundle connection information against each other and calculates the bundle diameter of the electrical wire bundle.