PURPOSE: To automatically correct misalignment of thermal ink jet print bars by electronic control.

CONSTITUTION: A diagnostic pattern comprising first and second lines is printed on a recording media (steps 106, 108). The diagnostic pattern is detected by a sensor, and the time elapsed between the forward end of the first line and the rearward end of the second line is measured (steps 110-116). When this time exceeds a set point, the steps 108-118 are repeated once more, and two elapsing times are compared with each other. Based on the comparison results, heating time interval of bank is regulated (step 124). The steps 108-116 are repeated with the regulated time interval, and when the elapsing time becomes shorter than the set point, correction processing is ended (steps 118, 126).