PURPOSE: To easily add ternary synchronizing signals to video signals with simple circuitry and to prevent the generation of noise.

CONSTITUTION: The ternary synchronizing signal addition circuit consists of a switching element 3 having a positive voltage terminal 6 and a switching element 4 having a negative voltage terminal 7 connected to a video input terminal 1 for an output amplifier 2. The switching element is turned on and off by controlling ternary synchronizing preparing signals S-1 and S-2 to add the ternary synchronizing signal consisting of positive and negative setting voltages in the blanking period of the video signal and the intermediate electric potential.