In a keyless system including a car-mounted device and a portable device carried by a user, the portable device detects the static state of the portable device for an arbitrarily set time period or longer with a static action detection section, and transmits a response signal serving as a response to a challenge signal transmitted from the car-mounted device on condition that the static state has been detected by the static action detection section. This can allow vehicle control such as locking and unlocking of a door of a vehicle through a simple action not requiring operation with a user's hand, and can prevent unnecessary vehicle control not intended by the user.