When a seal is inserted onto a shaft, load applied to a chuck is detected by a load cell, and based on the detected load, the turnover of a lip is judged. More specifically, an operation of inserting and pulling out the seal is performed several times on the shaft. Pull-out force is detected at the time of the last pull-out operation and insertion force is detected at the time of the last insertion operation. If the detected value is greater than a predetermined value, it is judged that the lip has been turned over. Also, a ratio between the aforementioned pull-out force and insertion force is computed. If the value of the computed ratio is less than a predetermined value, it is judged that the lip has been turned over.