PROBLEM TO BE SOLVED: To decide a communication timing for an orbital satellite being a hand-off destination by utilizing a relation between a Doppler frequency and a timewise rate of change of a radio wave propagation delay time so as to have only to conduct a simple arithmetic operation. SOLUTION: A mobile station 14 stores in advance a characteristic denoting a relation between a Doppler frequency obtained from a radio wave received from an orbital satellite and a timewise rate of change of a propagation delay time, obtains a timewise rate of change of a 1st propagation delay time with respect to an orbital satellite during communication at present and a timewise rate of change of a 2nd propagation delay time with respect to an orbital satellite being a hand-off destination on the basis of Doppler frequencies measured from radio waves received from the orbital satellite during communication at present and the orbital satellite being a hand-off destination and the characteristic above respectively, and calculates a timing of hand-off to the orbital satellite being a hand-off destination on the basis of a change in the propagation delay time that is obtained from the timewise rate of change of the 1st and 2nd propagation delay times and a time required for hand-off time after receiving the radio wave from the orbital satellite being a hand-off destination until the time of hand-off.
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