In a printer, threshold matrixes are prepared so that, in a color halftone image generated with the threshold matrixes, cell centers are arranged randomly, a direction where intervals of cell centers for cyan are maximum and a direction where intervals of cell centers for magenta are maximum are different, and the both directions are tilted relatively to a column direction corresponding to a scan direction of a head. A check pattern is recorded with the threshold matrix for each color component by the head, print densities in positions in a width direction of the check pattern, which corresponds to an arrangement direction of outlets in the head, are measured to acquire modification coefficients and then pixel values of an original image are modified by using the modification coefficients. This makes it possible to print a color halftone image with less graininess, without causing a beat pattern and streak unevenness.