Before figure data are expanded into a bitmap, a checksum is calculated in unit of bitmap data corresponding to a cell stripe of scanning over which continuous exposure is possible. When the checksum is calculated after expanding the data into the bitmap, the interim calculation result of refocus values is used. In exposure, exposing k number of sub rectangular areas by repeating a sub scanning k number of times, jumping a deflection by a main deflector toward an center of remaining sub rectangular areas whose number is (p-k) inside a main rectangular areas and exposing remained (p-k) number of the sub rectangular areas by repeating the sub scanning (p-k) times after the jumping is settled. In an amplifier & low pass filter for supplying a drive voltage to a sub deflector, the cutoff frequency is lowed during flyback in a sawtooth waveform without changing an amplification factor.