Abstract: The ground base station informs an in-level for the ground system, an out-level from the ground system, a timer value and an edge information indicating the edge of the service area, in addition to the broadcast information. It assumes that the mobile station MS moves from position 31 to zone 26 at the edge in the service area, and then moves to position 33. At the position 31, when the received field strength of the ground system decreases, the edge information is not detected, so that the mobile station MS does not switch the communication mode to the satellite system immediately by setting the timer value to three minutes. At position 32, the edge information is detected, so that the mobile station MS sets the timer value to one minute, and sets the out-level to a lower value than the general value. When the field strength is equal to or less than the out-level even after the passage of one minute, the mobile station MS switches the communication mode to the satellite system.