PURPOSE: To select and display only the direction of a required buoy among buoys of the same frequency by using a master and a slave counter and a control circuit which operates in following selectively a radio wave from the desired buoy.

CONSTITUTION: The master counter 1 counts input pulses 2 synchronously with rotational periods of antennas 9 and 10 and sends its output (a) to a latch 3. A receiver 20 has frequencies corresponding to the rotational periods of the antennas and outputs an output signal (b) which is in phase with the arrival direction of a radio wave. The slave counter 4 counts an input pulse train 2 and sends out a carry pulse C to output the counted value of the master counter 1 on a display device 6. The control circuit 33 synchronizes the carry pulse C of the slave counter 4 with the receiver output (b) and also opens a switch 21 to operate in following only the radio wave from the desired buoy.