In the case of a track inspection car operating on gradient line, a drift error in cross level irregularity occurs. In this paper, the cause of the drift error is made clear, and an easy compensation method of the error is suggested. As a result of analysis with co-ordinate transfer operation, the error is found to be caused by single axis type gyro equipment. The error arises under condition of applying pitch angle and yaw rate angle. The easy compensation method proposes, that the pitch angle is obtained from railway diagram, and the yaw rate angle from alignment. The availability of this method is revealed from practical use of the compensation method.