PURPOSE: To miniaturize the constitution as the whole by approximating the going optical path and the return optical path separated with a beam splitter to each other.

CONSTITUTION: A laser light LB emitted from a laser light source 20 is reflected on a plate-shaped optical element 23, which has a polarizing film 23a on the surface and consists of light-transmissive materials, and passes an optical system (means 4W6) including a quarter-wave plate 5 and is projected to a face 7 to be irradiated by the optical element 23, and a reflected laser light LBR is allowed to pass the polarizing film 23a and is reflected on a rear face 23b of the optical element 23 and is led to a light receiving part 24 provided near the laser light source 20. Consequently, the optical element 23 separates the laser light LB from the laser light source 20 and the reflected laser light LBR going to the light receiving part 24 from each other and approximates optical paths of the laser light LB and the reflected laser light LBR to each other. As the result, the constitution as the whole is miniaturized.