Violet narrowband light Vn and green narrowband light Gn produced by a light source device are supplied to a complementary color type endoscope, and simultaneously applied to an observation object. From a complementary color type imaging device, first mixed pixels and second mixed pixels, which sense both of the violet narrowband light Vn and the green narrowband light Gn, are read out. The light amount ratio Z of the violet narrowband light Vn to the green narrowband light Gn is set within a predetermined range based on an optimal light amount ratio Z0 at which the proportion P1 of a violet narrowband light Vn component within a first mixed pixel signal becomes equal to the proportion P2 of a green narrowband light Gn component within a second mixed pixel signal. This improves color separability of an endoscope system.