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In recent years, automatic transmissions have become widely used in cars. Compared to manual transmissions, automatic transmissions suffer from poor fuel economy. In order to overcome this disadvantage, a lock-up clutch system in the torque converter has been applied. When the rotating speed of the turbine approaches that of the pump, the input shaft is directly connected to the gear train through friction by means of the lock-up clutch. In the process of slipping at the lock-up clutch, frictional vibration referred to as shudder sometimes occurs. When shudder occurs, the power train, as well as the tires and the car seats, vibrates. Therefore, the shudder adversely affects passenger comfort. In the present study, experiments are conducted to analyze the shudder mechanism using a bench test apparatus and an actual vehicle. The characteristics of friction in the lock-up clutch is found to have a negative slope with respect to the relative slip velocity.