PURPOSE: To detect the no-light-out error of a laser beam, by providing an error detecting means, which generates a signal of the no-light-out error of the laser beam when a synchronizing signal is received from a light detecting means within the light-out period.

CONSTITUTION: When light emission from a laser diode 1 is stated and reflected light from a first mirror 21 is detected with a photodetector 23, an SOS signal I is obtained. A first counter 33 starts counting, and the light emission of the laser diode 1 is stopped. At the same time, counting of a counter circuit 34 is started, and a gate 35 is opened. When the counting is finished, the gate is closed. Meanwhile, the SOS signal I is directly sent to an error detecting circuit 36. When the laser diode 1 continuously emits the light during a light-out period, the SOS signal I based on the reflected light from a second mirror 22 is generated and reaches the error detecting circuit 36. The error detecting circuit 36 judges that the laser is not extinguished and outputs a no-light-out signal H of laser. An LBP process control part 37 stops each process of a laser printer at a suitable position. A character generator 12 stops the output of image data Q.