PURPOSE: To improve the efficiency of light focusing, by combining the a semi- cylindrical inner mirror and an optical transmission rod.

CONSTITUTION: A laser circuit slit 4 prolonged in parallel with an axis O is provided for a location apart from the center of an inner surface mirror 3 and an optical transmission rod 7 is provided for the position apart from the axis in parallel with the axis O toward the opposite direction to the slit 4. To decrease the uneven detection to the degree of no problem practically, it is desirable to take the width of optical diffusion provided over the entire length of the rod 7 at about 1/3 of the circumference. From the requirement of the distance between the rod circumferential positions A' and B' is about 1/12 to give the range of light reception angle of the reflected light in the range at least ±10°, it is desirable to take this width in the range of 1/3∼1/12 of the circumference.