Controller for electronic key system

A controller (3) for an electronic key system (1) including a portable device (2) and an engine (90). When the ID code of the portable device cannot be authenticated, the controller instructs the driver of the necessary operations. When a transmitter circuit (21) is transmitting a request signal but a receiver circuit (24) does not receive a signal including an ID code matching the ID code stored in a memory (28a), an authentication ECU (28) authenticates a transponder code of the portable device with a stored transponder code to enable the starting of the engine. The authentication ECU displays the necessary operations for performing transponder code authentication.