An endoscope system comprising: a light source apparatus configured to emit first and second narrow band illumination light (Gn, Vn) with different wavelength ranges in field sequential lighting, for endoscopic imaging; a multi-color image sensor, having plural pixels arranged on an imaging surface, configured to image reflected light from an object in a body cavity illuminated with said first and second narrow band illumination light, to output a pixel signal (P 1i, P 2i, Q 1i, Q 2i), wherein said plural pixels include first and second pixels that are different in spectral sensitivity of said pixels in relation to said first and second narrow band illumination light, wherein said first pixels have a lower spectral sensitivity than said second pixels to said first narrow band illumination light (Vn), and wherein said second pixels have a lower spectral sensitivity than said first pixels to said second narrow band illumination light (Gn); a gain corrector, supplied with said pixel signal by said image sensor, configured to perform first gain correction of multiplying said pixel signal of said first pixels by a first gain value (K 1i) to compensate for a difference in said spectral sensitivity of said first pixel from said second pixel to said first narrow band illumination light, and to perform second gain correction of multiplying said pixel signal of said second pixel by a second gain value (K 2i) to compensate for a difference in said spectral sensitivity of said second pixel from said first pixel to said second narrow band illumination light, wherein said first gain value (K 1i) and second gain value (K 2i) are determined according to a spectrum of light intensity of said first and second narrow band illumination light (Gn, Vn) and a spectrum of spectral sensitivity of said first and second pixels, and are constant without dependency upon a spectrum of spectral reflection of said body tissue, characterised in that said first and second narrow band light is narrow band violet light and narrow band green light, respectively, and wherein a wavelength range of each of said first and second narrow band illumination light (Gn, Vn) has a width equal to or less than 30 nm. a light source apparatus configured to emit first and second narrow band illumination light (Gn, Vn) with different wavelength ranges in field sequential lighting, for endoscopic imaging; a multi-color image sensor, having plural pixels arranged on an imaging surface, configured to image reflected light from an object in a body cavity illuminated with said first and second narrow band illumination light, to output a pixel signal (P 1i, P 2i, Q 1i, Q 2i), wherein said plural pixels include first and second pixels that are different in spectral sensitivity of said pixels in relation to said first and second narrow band illumination light, wherein said first pixels have a lower spectral sensitivity than said second pixels to said first narrow band illumination light (Vn), and wherein said second pixels have a lower spectral sensitivity than said first pixels to said second narrow band illumination light (Gn); a gain corrector, supplied with said pixel signal by said image sensor, configured to perform first gain correction of multiplying said pixel signal of said first pixels by a first gain value (K 1i) to compensate for a difference in said spectral sensitivity of said first pixel from said second pixel to said first narrow band illumination light, and to perform second gain correction of multiplying said pixel signal of said second pixel by a second gain value (K 2i) to compensate for a difference in said spectral sensitivity of said second pixel from said first pixel to said second narrow band illumination light, wherein said first gain value (K 1i) and second gain value (K 2i) are determined according to a spectrum of light intensity of said first and second narrow band illumination light (Gn, Vn) and a spectrum of spectral sensitivity of said first and second pixels, and are constant without dependency upon a spectrum of spectral reflection of said body tissue, characterised in that said first and second narrow band light is narrow band violet light and narrow band green light, respectively, and wherein a wavelength range of each of said first and second narrow band illumination light (Gn, Vn) has a width equal to or less than 30 nm.