PURPOSE: To make the alignment in axis of optical fibers easy with an easy operation arranging two prisms vertically to a beam in the state, where they are rotated at 180° to each other, while satisfying specific conditional formulas between incident and exit angles and the refractive index.

CONSTITUTION: An optical beam L of a semiconductor laser 1 is converted to a parallel light having a longer axis a1 and a shorter axis b1 by a collimator lens 2; and when this parallel light is made incident vertically on a prism 3, an exit light L2 is inclined at an angle γ to the axis. When the exit light L2 is made incident vertically on a prism 4 which is rotated at 180° around the vertical axis with respect to the prism 3, an exit light L3 is inclined at the angle γ. If angles α and β are set so that a formula is satisfied, a longer axis a3 of the beam L3 is equal to a shorter axis b3. Thus, the elliptic beam is shaped to a circular beam and axes are made parallel to make the optical fiber inclining operation or the like unnecessary, and the alignment is performed easily.