PURPOSE: To provide an image synthesizer for performing the color/black-and- white conversion in the optional area of an image programmably at a real time when a moving image, etc., is displayed by using a computer.

CONSTITUTION: This synthesizer is the image synthesizer dividing a display picture to plural areas, displaying a first image on a first area and simultaneously displaying a second image on the other area, and is provided with keep lanes 8a, 8b keeping the data showing the synthesis positions of the first and the second images, a data expansion part 20 expanding the data showing the compressed synthesis positions and storing the data in the keep lanes 8a, 8b, an image sysnthesis part 7 switching the first and the second image based on the data outputted from the keep lanes 8a, 8b and generating a synthesis image and a transfer control part 10 respectively outputting address control signals corresponding to the syntheses image to the keep lanes 8a, 8b and the image synthesis part 7.