The invention provides a mobile device system and a mobile device that can flexibly respond to situations where the user carries a different combination of mobile devices on different days, and can prevent the mobile devices from being misplaced. The mobile device system includes three or more mobile devices, wherein each mobile device includes a device detection unit for periodically detecting other mobile devices located within a predetermined range of the mobile device, a function-restricting unit for restricting a predetermined function, and a control unit, and when two or more mobile devices of the other mobile devices detected by the device detection unit are no longer detected, the control unit operates the function-restricting unit.