An image data processor comprising an image signal receiver, a histogram generator, a gain calculator, an amplifier, and a signal feeder, is provided. The image signal receiver receives an autofluorescence image signal. The autofluorescence image signal is generated by an imaging device when the imaging device captures an autofluorescence image. The histogram generator generates a histogram of luminance in the autofluorescence image based on the autofluorescence image signal. The gain calculator calculates a gain based on the histogram and a predetermined luminance value. The amplifier amplifies the autofluorescence image signal by the gain. And then the amplifier generates an amplified autofluorescence image signal. The signal feeder outputs the amplified autofluorescence image signal to a monitor. The monitor displays an amplified autofluorescence image.