1. An image processing device, comprising: an obtainer that obtains color endoscopic image data; a color space converter that converts, by a processor, a color space of the color endoscopic image data to one of an HSI color space based on hue, saturation and intensity, and an HSV color space based on hue, saturation and value; a lesion pixel judger that judges, by a processor, for each of pixels constituting a color endoscopic image, whether or not each pixel is a lesion portion based on the hue and the saturation in the converted color space; an overlay processor that executes an overlay process which changes a color of the pixel judged to be the lesion portion by the lesion pixel judger, the overlay processor changes the color of the pixel judged to be the lesion portion by enhancing a predetermined color component of the pixel judged to be the lesion portion and not enhancing the predetermined color component of a pixel not judged to be the lesion portion; and an overlay image display that displays an overlay image obtained by subjecting the color endoscopic image to the overlay process, wherein each of the obtainer, the converter, the judger the overlay processor and the display comprise an image processing circuit.