PURPOSE: To reduce the reception signal pull-in time by giving frequency sweep range information to a control circuit while the frequency range is limited to a frequency range so as to absorb the maximum deviation of a transmission frequency after change with respect to the transmission frequency before change when the transmission frequency information of an opposite station transmission system has a change.

CONSTITUTION: In the case of changing a reception channel F1 into a reception channel F2, when the reception channel F2 is inputted from a reception channel output circuit 11, a control circuit 8 changes the reception channel F1 to control a frequency synthesizer of a 1st frequency conversion circuit 2 with a control signal 9 thereby obtaining the reception channel F2. When a synchronization establishing signal 12 is asynchronous, a frequency fs (Hz) taking the long period stability of a local oscillator included in an opposite station, a space station and its own station into account is selected as the frequency sweep range in order to lock a reception signal of the reception channel F2, the variable local oscillator included in the 2nd frequency conversion circuit 4 is not controlled but the sweep range based on the difference between frequencies of the transmission system between the opposite stations is selected.