To discover early a deficiency such as turn-up (tearing) of a lip part or the like when press fitting oil seal.

After press fitting the oil seal r between transmission case C and a shaft S by a press-in head 2, the shaft S is rotated by a rotation means 3. Simultaneously, pressurized air is supplied from the lower side of the oil seal r by a pressurized air supply means 4, and the difference between the pressure in a detection part 7 of the press-in head 2 and the outside air pressure is detected by a differential pressure detection sensor 16 of a pressure change detection means 5, to thereby discover a pressure leak from the oil seal r.