Linearly polarized light propagating through an ionized region, with a nonzero component of the magnetic field parallel to the direction of propagation, experiences a rotation of the polarization plane due to the Faraday effect. Measurement of the rotation angle of the polarization plane can provide information regarding the electron density and discharge current of discharges in air. In this experiment, the rotation of the polarization plane of linearly polarized laser light propagating through an ionized region created by a spark discharge in air was examined. The rotation angle of the polarization plane was estimated from the change in the intensities of the mutually perpendicular polarization components. In the experiment using an air gap of length 1 m, the rotation angle was detected only when the laser beam intersected or passed through the immediate vicinity of the path of the spark discharge, and the measured rotation angle was 0.54 degrees. In another experiment using an air gap of length 0.77 m, the polarization rotation was confirmed to exhibit a temporal variation similar to the discharge current waveform. Furthermore, the effects due to the neutral density variation accompanying the discharge were shown to have no effect on the polarization rotation measurement. Copyright © 2010 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.