A slip control system for a torque converter with a lockup clutch comprises a controller that outputs a slip control signal to a slip control actuator of the lockup clutch to adjust an actual slip rotation speed of the torque converter at a target slip rotation speed. The controller is coupled to sensors for detecting information indicative of a condition of a drive system including the torque converter. The controller decides an operating state of the torque converter at time when the slip control is started, and selects one of initial valves for a slip command signal corresponding to the slip control signal according to the decided operating state of the torque converter at the start of the slip control. This arrangement functions to prevent troubles including shocks due to shortage of slippage in the torque converter and generation of radial slippage of the torque convert from generating at a time staring the slip control.