A car sharing system includes a car sharing device. The car sharing device includes a key function unit and a user authentication function unit. The key function unit allows a device of a vehicle that is shared with multiple people to operate by performing ID verification through a process similar to an electronic key of the vehicle. When the device of the vehicle is operated with a mobile terminal, the user authentication function unit obtains key information from an external device via the mobile terminal and performs authentication of the key information. When authentication of the key information is accomplished and use of the vehicle is within a reservation time, the user authentication function unit validates the key function unit. The key information is generated as information that is permitted only a temporary use.