PURPOSE: To achieve a smaller size and a less weight of the apparatus and a shorter assembling time by arranging a beam shaping prism, a pluralization beam splitter, a triangular prism, a half mirror, a total reflection mirror, a Savart plate, a 4-split prism, a photodetector of a type of more than 5 divisions and a photodetector of a type of more than 1 division.

CONSTITUTION: A semiconductor laser 1 as light source, a collimation lens 2, a beam shaping prism 3, a polarization beam splitter 25, a triangular prism 6, half mirrors 5 and 15 and a total reflection mirror 22 are arranged. Also arranged are a Savart plate and a 4-split prism 23 which are made to join on two outgoing surfaces from which a luminous flux is emitted in a direction orthogonal to an optical axis of a flat convex lens of the above half mirrors, a photodetector 24 of a type of more than 5 divisions disposed in front of the 4-split prism 23 and a photodetector 21 of a type of more than 1 division disposed in front of the Savart plate 20. This achieves a smaller size and a less weight of the apparatus and a shorter assembling time thereof with the curtailment of the prisms and photodetectors.