PURPOSE: To attain the miniaturization of a whole device irrespective of that the device is provided with functions of detecting the positional deviation of a laser beam and automatically adjusting the set position of a scanning mirror.

CONSTITUTION: The device is constituted of scanning mirrors 3a and 3b for separately polarizing the laser beam emitted from a laser oscillator 1 in X-axis and Y-axis directions, position detecting lasers 5a and 5b for irradiating with the laser beam a surface opposite to the surface which is irradiated with the laser beam emitted from the laser oscillator 1 of each mirror 3a and 3b, line sensors 6a and 6b for receiving the laser beam reflected by respective scanning mirrors 3a and 3b after being emitted from respective position detecting lasers 5a and 5b, and then, detecting the positional deviation of the laser beam, and a scanning mirror control circuit 7 for automatically adjusting the set position of each scanning mirror 3a and 3b based on the detection results of the line sensors 6a and 6b.