To input a plurality of image signals from a plurality of PCs to an image display device to display a plurality of images without increasing the number of resolution conversion means.

In a period when a first PC 1 does not agree with an actual resolution of a display part 10 from the power-on time to accomplishment of the final number of display pixels, a resolution conversion circuit 7 is adapted to an image signal of the first PC 1. An image of the first PC 1 is displayed, and the resolution conversion circuit 7 is adapted to an image signal of a second PC 2 after the first PC 1 accomplishes a preliminarily set actual resolution of the display part 10. Thus the size of an image of the second PC 2 can be changed by wiping display or picture-on-picture display of the image of the second PC 2 without increasing the number of resolution conversion circuits while displaying the image of the first PC on the display part 10 with the actual resolution.