Print head for black material is arranged such that the black material is to be ejected later than cyan, magenta, and yellow materials on both of forward and backward strokes, and the print heads for the cyan, magenta, and yellow materials are arranged in tandem in a main-scanning direction. The first processing unit converts input image data into CMY image data and then performs black generation and undercolor removal on the CMY image data, to obtain CMYK image data. The second processing unit converts CMY image data excluding K image data from the CMYK image data into image data for use in printing in the backward direction. The on/off switching unit switches between causing and not causing the second processing unit to covert the CMY image data depending on which one of printing in the forward stroke and printing in the backward stroke is to be performed.