In a wet-clutch break-in method, a torque converter assembly 1 incorporating a wet-type lockup clutch 8 is clamped between a lower-jig work receiving member 31 and an upper jig face plate 51, and the torque converter assembly 1 is filled with torque converter fluid supplied through a passage 34 extending inside an output shaft 26. Then a lockup-on pressure is supplied through a passage 36 extending between the output shaft 26 and a stator shaft 30 to bring a clutch disc 13 and a front cover 2 of the torque converter assembly into friction contact. The friction contact surfaces of the clutch disc and the front cover are made in the break-in process by rotating the upper jig face plate 51 by rotation of a motor transmitted by an input shaft 50 while stopping the output shaft 26 from rotating. Vibration occurring during the break-in is monitored by a vibration pickup disposed on the output shaft, and the motor rotation number or the lockup-on pressure is accordingly controlled, to avoid resonance of the break-in apparatus.