With a device that connects a processor device to electronic endoscopes having CCDs with different numbers of pixels, for example, 410, 270, or 190 thousand pixels mounted thereon, all the CCDs are driven at a frequency corresponding to 410 thousand pixels. Further, an information amount converting circuit enlarges an image obtained by the CCD with 270 or 190 thousand pixels in a horizontal direction and a vertical direction by using pixel interpolation. That is, data on a pixel to be interpolated is calculated by averaging pixels around this pixel, and enlarged binary data on this target pixel is obtained. A weighting factor is applied to this binary data, and the resultant data is added to the target pixel data. This process forms an image with an aspect ratio of 4:3.