PURPOSE: To surely detect defective printing by counting horizontal synchronizing signals in each printing line, and when a succeeding signal is not inputted within a prescribed time, judging defective printing by an interruption signal outputted in each arrival at a prescribed count value.

CONSTITUTION: When horizontal synchronizing signals HS corresponding to a prescribed number are not outputted from a laser printer body 11 and the count value of a counter 61 does not reach a prescribed value, printing operation is waited until the lapse of a specified time. When an interruption signal IS is inputted from an interruption signal generating circuit 62 after the lapse of the specified time, the output of the signals HS of the prescribed number from the body 11 is judged as a normal printing state and an interruption signal clearing circuit 63 outputs a clear signal CS to clear the circuit 62 and the count value. At the time of predicting the input of the signal IS, said operation is repeatedly executed. When a succeeding signal IS is not inputted, whether residual data are normally printed or not is judged.