In the combination of a wide dynamic range by combining a first image and a second image of different light exposure conditions, pixel values at which cumulative frequency proportions (B) become first and second reference values are generated as first and second index values (Sa, Lb) with regard to the first and second images, and first and second light exposure amounts (ES, EL) are controlled under first and second target conditions that the first and second index values (Sa, Lb) have predetermined relations with first and second target values (Sat, Lbt), respectively. It is possible to shorten a time taken until convergence of the light exposure amount in relation to a change of brightness of a subject.