A polarization-maintaining optical fiber takes the attitude of the rotation angle position establishing the maximum extinction ratio in a condition where the polarization-maintaining optical fiber receives no load. Under this situation, the first characteristic curve is derived to present the values of the extinction ratio for various relative rotation angles between the light source and the polarization-maintaining optical fiber establishing the maximum extinction ratio in response to the application of load. The second characteristic curve is likewise derived to present the values of the extinction ratio for various relative rotation angles between the light source and the polarization-maintaining optical fiber establishing the minimum extinction ratio in response to the application of load. The predetermined rotation angle position is thus defined based on the intersection point between the first and second characteristic curves. The extinction ratio needs not be measured for a large number of values of the load.