PURPOSE: To improve the angle setting precision of a polarizer and improve the measuring precision of a current.

CONSTITUTION: A light application device is provided with a laser 1 serving as a light source, a polarizer 2 converting the light from the laser 1 into the linear polarized light, a Faraday element 5 fed with the linear polarized light, an analyzer 3 dividing the light transmitting the Faraday element 5 into the X-constituent light and the Y-constituent light, photo-diodes 6x, 6y converting the X-constituent light and the Y-constituent light into electric signals, AGC amplifiers 7x, 7y amplifying the electric signals converted by the photo-diodes 6x, 6y, an analog divider 8 dividing the difference between the output signals of the AGC amplifiers 7x, 7y with the sum of the output signals, and a distortion factor gauge 9 measuring the distortion of the output of the analog divider 8. The Faraday element 5 is arranged in the magnetic field formed by the sine wave alternating current.