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An insulating oil/pressboard (PB) composite insulation is generally utilized in oil-immersed power transformers. In such a transformer, charges accumulate at oil/PB interface which changes the electric field distribution depending on time and oil condition. In this paper, we report on directly measured dc electric field in oil/PB composite insulation system using the Kerr electro-optic field measurement technique. From measurement results, we propose a physical model for the charge behavior. We have calculated an electric field distribution using finite element method (FEM) based on the charge movement. We compared the results of measurements and calculations and confirmed that the proposed charge behavior model explains well the experimental results in the case of the polarity reversals as well. As a result, a significant validity of the charge behavior model was confirmed.