In an electronic key system for a motorcycle including the control unit mounted on a specific vehicle and a mobile transmitter-receiver carried by a user, the specific vehicle includeing an openable and closable seat on which the user sits, and is provided with a start switch used for starting the control unit in the seat. More specifically, the start switch is installed on, for example, the left side surface inside the user's seat at the position corresponding to the portion which the user may touch when opening the seat. An operating element of the start switch opposes the back surface of the superficial skin on the left side of the seat.