A light source unit emits multi-colored light. The amount of light of each color is controlled so that the multi-colored light has a first light emission ratio. A color image sensor images an object and outputs a multi-colored image signal. Again coefficient calculator calculates provisional gain coefficients based on the multi-colored image signal. A noise evaluation value calculator calculates a noise evaluation value based on the provisional gain coefficients and set color correction coefficients. A judging section judges whether or not the noise evaluation value is an allowance or less. In a case where the noise evaluation value is judged to be the allowance or less, a coefficient determiner determines the provisional gain coefficients as set gain coefficients, and a light emission ratio determiner determines the first light emission ratio of the multi-colored light as a set light emission ratio.