Abstract: A lock-up control system for an automatic transmission of an automotive vehicle controls operation of a lock-up clutch for mechanically engaging the automatic transmission with the engine to enable transmission of a driving force output from the engine to the automatic transmission. An engaging force of the lock-up clutch is controlled in an predetermined operating region of the automotive vehicle, which is determined based on operating conditions of the automotive vehicle and the engine, in such a manner that an amount of slip of the lock-up clutch becomes equal to a desired value. An engaging force control amount for controlling the engaging force of the lock-up clutch is learned when the automotive vehicle is traveling in the predetermined operating region with the automatic transmission being in a predetermined speed position, and a learned value thus obtained is stored.