PURPOSE: To prevent the deviation of a display picture by measuring a pulse width of a horizontal synchronizing signal separated from a 2nd video signal read from a memory, detecting a difference to a normal synchronizing signal width, setting a front edge of a reference decoding synchronizing signal and outputting the result as a 3rd video signal.

CONSTITUTION: A horizontal synchronizing signal separator circuit 21 separates a horizontal synchronizing signal I of a video signal output H of a D/A converter 20 and gives the signal to a width detection circuit 22. The width detection circuit 22 detects a time width of the horizontal synchronizing signal I and gives a control signal according to the width to a phase adjustment circuit 23. The phase adjustment circuit 23 receives the control signal to output a reference composite synchronizing signal J equal to a time width of the horizontal synchronizing signal I by adjusting the time width of the horizontal synchronizing signal in the reference composite synchronizing signal G generated in advance at its front ridge. A replacement circuit 24 replaces a composite synchronizing signal of a waveform H with the composite synchronizing signal J generated separately while matching the trailing edge.