PURPOSE: To take out a Shiroma-key signal (a kind of chroma-key about background) from a luminance signal, and to obtain a correct composite signal, by constituting a composing circuit including an average value output means which obtains an average value of the luminance signal of two frames.

CONSTITUTION: An A/D converter 3 digitizes the video signal, which is the output of a VTR1, by the timing signal of a timing oscillating circuit 6 and not only records the signal of one-frame components in a memory 4 but also records the signal of next one-frame components in a memory 5. When both video signals are recorded in memories 4 and 5, an adding circuit 7 adds both video signals, and the addition output is reduced to 1/2 by a 1/2 circuit 8 and is recorded in the memory 4. Next, the A/D converter 3 digitizes the output of a image pickup machine 2 and records one-frame components in the memory 5. Meanwhile, a Shiroma-key signal is detected from the video signal of the memory 4 by the circuit 7 and is inputted to a synthesizing circuit 10 together with video signals of memories 4 and 5, thus synthesizing videos.