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The investigations of the dc electric field distorted by space charges have been made by means of the optical method using a Pockels device. The Pockels device was rotated to eliminate field disturbances caused by surface charges accumulated on it. The present method has the advantage of directly measuring the electric field in a dc corona discharge. The method was successfully applied to the measurement of the electric‐field distribution in a needle‐to‐plane gap of 10 cm with a positive dc corona. The field distortion was clearly observed in a wide range of applied voltages between 30 and 98% of the breakdown voltage 52 kV. It was found that the electric field inside the corona streamers was 3.5 kV/cm, while 14 kV/cm near the tip of the streamers. Furthermore, the experimental results were theoretically confirmed at the relatively low voltage.