A mobile device system that includes left and right earphones, and a mobile device connectable to the left and right earphones. Each of the left and right earphones includes at least one of an acceleration sensor and a geomagnetic sensor. The mobile device includes a controller that monitors a wearing state of the individual left and right earphones by a user on a basis of output from the least one of the acceleration sensors and the geomagnetic sensors, and controls a behavior of the mobile device connected to the left and right earphones according to the monitored wearing state of the left and right earphones by the user.