A car sharing system includes a car sharing device that actuates an onboard device by performing communication with an electronic key system through ID verification when a user operates the onboard device in an authentication completion state in which key information has been authenticated through wireless communication with the portable terminal, a radio wave checker that checks radio waves transmitted from the electronic key system in accordance with a state of a vehicle power supply when the onboard device is operated with the portable terminal and a key control request corresponding to the operation with the portable terminal is transmitted from the portable terminal to the car sharing device so that the car sharing device actuates the electronic key system, and a warning controller that issues a warning indicating that one has forgotten to stop the vehicle power supply in accordance with a checking result of the radio wave checker.