is also connected to the set terminal 511 s of the rs flip flop 511 therefore in response to the received light pulse first arriving after operation of the gate circuit 503 the rs flip flop 511 is set and the output of the flip flop 511 operates the gate circuit 515 under this condition the rotational speed signal 517 for driving the motor is applied to the counter 512 through the switch 518 and the gate circuit 515 so that the counter 512 counts the rise or fall edges of the motor driving rotational speed signal 517 the count of the counter 512 is compared with the set value of the comparator circuit 513 in the next stage when the count of the counter 512 exceeds the set value of the comparator 513 the output of the comparator circuit 513 is reversed this signal is applied to the reset terminal 512 r thus resetting the count of the counter 512 to zero the output of the comparator circuit 513 is further reversed to the original state in this way the comparator circuit 513 produces a pulse when the count of the counter 512 exceeds the set value of the comparator circuit 513 the output pulse of the comparator circuit 513 is applied as a latch timing signal to the latch circuit 505 through the or gate 514 firstly as a.