PURPOSE: To generate prediction gate timing which is synchronized with a period pulse group signal.

CONSTITUTION: An input pulse group signal 2 is inhibited by an input pulse group inhibition gate 1 on the basis of a pulse group prediction gate signal 3. The output signal of the input pulse group inhibition gate 1 is used as the reset signal of an UP/DOWN counter 4. A coincidence detecting circuit 7 detects whether or not a limit value 8 and the output signal of the UP/DOWN counter 4 coincide with each other and in the case of coincidence, an UP/DOWN flip- flop 9 is reset. When the output of the UP/DOWN counter 4 all becomes zero, an UP/DOWN flip-flop 9 is set. Consequently, the UP/DOWN control of the UP/DOWN counter 4 is performed and a comparator 5 compares in magnitude the output signal of the UP/DOWN counter 4 with a prediction gate pulse width set value 6 to generate the pulse group prediction gate signal 3.