PURPOSE: To simply and highly accurately measure the polarization dependency of optical parts without using expensive optical parts by rotating respective connectors connected to both the ends of a constant polarization optical fiber around a light advancing direction and controlling the polarizing direction of light made incident upon the optical parts.

CONSTITUTION: A connector 3a to be roughly and finely rotated is connected to a receptacle 2 to be the output terminal of a light source 1 and a rotatable connector 3b connected to the other end through the constant polarization optical fiber 4 is connected to a sensor 6a for an optical power meter 5. At first the connector 3a connected to the light source 1 is rotated and fixed at a position maximizing the output of light projected from the fiber 4. Then the connector 3b is reconnected to an incident end connector 8a of the optical parts 7 to be measured and rotated by a prescribed angle to measure the optical output projected from the optical parts 7. Thus, the polarization dependency of various optical parts can easily be measured without using a large optical system.