A width A of the scanning area of the ink ejection orifices and a width B of the scanning area of the reacting liquid ejection orifices are respectively set as A=(n-a).times.p and B=n.times.p, while amount of the feeding of the printing sheet during each scan corresponds to the width of the scanning area of the reacting liquid ejection orifices, that is, A=(n-a).times.p. With this system, the width of the scanning area, wherein ejection of the reacting liquid precedes ejection of the ink, is made shorter by C=a.times.p than the width of following scanning area; the scanning area having the width C is scanned two times by the row of the ink ejection orifices, and the thinning process is applied to this area having the width C.