The present invention produces halftone dots which shows smaller fluctuations in size and shape irrespective of variation of the positional relation between the address of screen pattern data and the recording pixels. Displacement F1 through F4 are computed at vertices of a halftone dot region VP1 through VP4. Each of the vertices is shifted to a lattice point of the scanning coordinate systems, and the X-Y coordinates of each lattice point in the scanning coordinate systems are corrected accordingly. The displacement of an arbitrary lattice point A is interpolated from the displacement F1 through F4 on the basis of the distance between each vertex and the lattice point A. As a result, the positional relation between the address of the screen pattern and the position of the recording pixels is maintained constant in the vicinity of the vertices, and this reduces the fluctuations in size and shape of the halftone dots. The center of each halftone dot region can be shifted to a lattice point in the scanning coordinate systems.