PURPOSE: To simplify the constitution of the display color expanding device by constituting a device which switches image data from plural image memories and outputs them after color expansion by using one lookup table circuit.

CONSTITUTION: The image data D1 and D2 from the image memories 11 and 12 are multiplexed by a multiplexing means 21 into one output cycle and then the one lookup table circuit 22 performs the color expansion. Output image data determining means 25 and 26 determine image data to be outputted as image data in output cycles according to the transparent color information of the color-expanded and multiplexed image data LM, and image data selecting means 23 and 24 select the determined image data among the multiplexed image data and output them. Consequently, even when there are plural memories 11 and 12 provided as image memories, the color expansion processing is performed by using the one lookup table circuit 22, whose part can be simplified.