A plurality of mobile devices interoperate without configuration operation. A mobile device system including a plurality of mobile devices is provided. Each mobile device is operable in one of an independent-operation mode in which the mobile device operates with priority over the other mobile devices and a dependent mode in which the mobile device operates dependently on another mobile device and includes a control unit having a start-up-order detection unit configured to detect the start-up order of the mobile device with respect to the other mobile devices. A first mobile device operates in the independent-operation mode when the start-up order of the first mobile device detected by the start-up-order detection unit is earlier than the start-up order of a second mobile device and operates in the dependent-operation mode when the start-up order is later than the start-up order of the second mobile device.