PURPOSE:To keep a quantity of light constant as passing through a polarizer by converting all optical paths containing a polarizer and a depolarizer to an optical fiber to add another depolarizer in a single-mode optical fiber from a light emitting element to the polarizer. CONSTITUTION:A light emitting element 1 is a light source which emits a monochromatic light and employs a laser diode or the like with a coherent length shorter. Besides a depolarizer 3 normally provided, a depolarizer 2 is disposed in the course of a first fiber optical path 11 of a single mode optical fiber for connecting the light emitting element 1 and a fiber type polarizer 9. Then, light emitted from the light emitting element 1 is a linearly polarized light, depolarized with the depolarizer 2 and reaches the polarizer 9, where it is turned to a linearly polarized light in a certain direction. As the light is depolarized once, an amplitude of the light when turned to the linearly polarized light with the polarizer 9 is constant irrelevant to a bearing of the light emitting element. This eliminates the need for adjusting axes between the light emitting element 1 and the polarizer 9 and there is no effect on the light as the light is depolarized even when a polarization plane is rotated in an optical fiber as caused by external forces and temperatures.
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