Line patches m and solid patches s are printed with a subject image, each line patches having at least 200 lines per inch and a duty ratio of at least 60%, to detect densities Dm and Ds. A water coefficient W (=Dm/Ds) is calculated from the detected densities Dm and Ds. Whether dampening water is fed at a proper rate or not is determined based on correlation data stored beforehand and showing a relationship between the feed rate of dampening water and the water coefficient W. The feed rate of dampening water is controlled to be a proper rate.