PURPOSE: To obtain a key signal detecting circuit in a chromakey system with a simple circuit and accurate color adjustment by detecting a chromakey signal by a color difference signal.

CONSTITUTION: The color difference signal (B-Y) as shown in (b-1) is waveform- shaped by a comparator and a waveform-shaped signal (B-Y)' waveform-shaped as shown in (b-2) is fed directly to an AND gate 2. On the other hand, a color difference signal (R-Y) shown in (b-3) is waveform-shaped by a window comparator and a waveform-shaped color difference signal (R-Y)' as shown in (b-4) is obtained and this color difference signal (R-Y)' is inverted and fed to an AND gate 2. Through the constitution above, the color difference signals (B-Y)' and (R-Y)' are fed to the AND gate 2, the inverted color difference signals (R-Y)' and (B-Y)' are ANDed at the AND gate 2 and the key signal is outputted from the AND gate 2 only when the (B-Y)' is at a low level and the (R-Y)' is at a high level.