This paper analyses the effects of schooling, outside work experience and job tenure on human capital formation, using the personal history data of white collar employees of Mitsubishi Zaibatsu in prewar Japan. For all samples including both engineers and clerks, the rate of return to schooling was 4.19%, which was higher than those to outside work experience and tenure. In case we allow for difference in the effects of schooling, outside work experience and job tenure between engineers and clerks, the rate of return to schooling was 5.11% for engineers, while it was 2.61% for clerks. It is notable that for clerks the rate of return to schooling was lower than that to job tenure. These results imply that Mitsubishi differentiated the systems of human capital formation between engineers and clerks. That is, while schooling was the central measure for human capital formation for engineers, on-the-job-training inside the firm was that for clerks.