A printer controller 41 sets compensation values, representing the positional deviations of LED array chips 31, in a delay circuit section 68 via shift register 61 and a latch circuit 63. The shift register 69 sequentially acquires bit map data from the printer controller 41, and supplies the acquired bit map data to an AND gate 66 through the latch circuit 62. Upon receiving a strobe signal from the printer controller 41, the delay circuit section 68 supplies the received strobe signal to the AND gate 66 after a delay time corresponding to a preset compensation value has lapsed. The AND gate 66 generates drive signals each being the logical product of the input bit map data and the strobe signal, and supplies them to the LED array chips 31 through a buffer 67. The LED array chips 31 make LEDs emit light at the timings corresponding to the positional deviations so that images are formed without a positional deviation. At least one of the LED array chips is unaligned with adjacent ones of the LED array chips and preset compensation values are obtained by interpolation based on detected positional deviations of sampled ones of the LED array chips.