A processing apparatus processes an image signal generated by pixels that have received light from an object irradiated with first wavelength light. The processing apparatus includes a processor having hardware. The processor is configured to: extract first and second image signals from the image signal of one frame generated by the pixels, the first image signal being generated by a pixel that has received the first wavelength light, the second image signal being generated by a pixel that has received second wavelength light that is fluorescence emitted in response to the first wavelength light； estimate an image signal to be generated by the pixel that has received the second wavelength light, based on the first image signal； and calculate a difference, on a corresponding image signal portion, between the second image signal and the estimated image signal, thereby to obtain a differential image signal.
