An image processing device includes: a memory; and a processor comprising hardware, wherein the processor is configured to: receive agent observation image information including information of a plurality of pixels obtained by capturing an image based on fluorescence from a subject administered with an agent that emits fluorescence upon being irradiated with excitation light in a predetermined wavelength band; amplify pixel values of the plurality of pixels by executing first gain processing on the agent observation image information; and reduce pixel values of pixels lower than a predetermined threshold by executing reduction processing on the agent observation image information after the first gain processing.