1. An image processing device, comprising: an obtaining unit configured to obtain color endoscopic image data; a color space conversion unit configured to convert 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 hue, saturation and a value; a lesion pixel judgment unit configured to judge, 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; an overlay processing unit configured to execute an overlay process which changes a color of the pixel judged to be the lesion portion by the lesion pixel judgment unit; and an overlay image display unit configured to display an overlay image obtained by subjecting the color endoscopic image to the overlay process.