PURPOSE: To surely detect synchronism, by transmitting a synchronization detecting signal, only when outputs of the 1st and 2nd counters are coincident with a set value corresponding to the number of unit bits determined at the synchronism section of a disc.

CONSTITUTION: A signal A at an input terminal 3 outputs a differentiation pulse P1 at the 1st differentiation circuit 2. The 2nd differentiation circuit 8 outputs a differentiation pulse P2. Counters 12 and 13 are cleared at the pulses P1 and P2 and counts a clock pulse CP. The 1st shift register 14 fetches a value of the counter 12 at the trailing of the signal A in parallel and is shifted. Thus, a value of the register 14 represents the number of bits at each one period of the signal A. This is the same for a shift register 15. Signals at 1,0 periods from the registers 14 and 15 are converted and outputted into a decimal number at decoders 16 and 17. Thus, the synchronism detection can be performed by taking out the consecutive part of 0 and 1.