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.