PURPOSE: To obtain video muting operation and color killer operation in addition to a normal mixing operation by constituting a differential amplifier in such a way of an emitter resistance and a collector resistance are used in common with a transistor for inputing a chromaticity signal and that for inputting a luminance signal, and a collector of an emitter grounding transistor and that of a base grounding one are commonly connected.

CONSTITUTION: When a color killer mode is selected, a transistor Q2 is brought into a disconnected state since the base of the transistor Q2 serves as an earth potential. As the output of a comparator circuit 7 in a color killer circuit 8 goes to a low level, a transistor Q4 is made nonconductive, while a transistor Q3 is conducted. Accordingly, since the transistor Q3 goes to operable instead of the transistor Q2, the supply of a chromaticity signal is shut off. However, the DC component of base voltage of a transistor Q5 is held to the same value as in a normal mixing mode. Consequently, video muting and color killer operations are made possible in addition to the normal mixing operation only by changing the input level of chromaticity signal.