To provide a method for setting the specification of on-vehicle electronic control units capable of reducing the kinds of the on-vehicle electronic control units and dispensing with any exclusive signal line for setting the specification.

An on-vehicle electronic control unit 1 detects a predetermined operation by a worker, outputs a predetermined control signal to a corresponding device to be controlled, allows the device to be controlled to perform the predetermined operation (S105: operation check step), temporarily determines presence/absence of the device to be controlled which performed the predetermined operation (S107), and stores a result of the temporary determination in a memory unit 15 (S109: storage step). If it is temporarily determined that the device to be controlled is present, it is set that the specification includes the device to be controlled. The on-vehicle electronic control unit 1 detects that the control signal to instruct the specification decision is input, and an ignition switch 17 is turned off, and decides the result of the temporary determination stored in the memory unit 15 as the specification of the on-vehicle electronic control unit 1 (S113: specification decision step).