Image processing device, endoscope apparatus, information storage device, and image processing method

An image processing device includes a processor including hardware, the processor being configured to: acquire an input image; detect an arc curve in the input image; determine a change in a luminance of a pixel in a direction from an inner side toward an outer side of the arc curve; detect, as a representative bubble region, a region including the arc curve involving a change determined to be an increase in the luminance in the direction from the inner side toward the outer side; and extract a bubble region, based on the representative bubble region.