Abstract: Method and apparatus for carburizing, quenching and tempering workpieces, by which the step of carburizing can be incorporated into a machining and manufacturing process to be performed by an on-line machining and manufacturing system as one stage thereof, and by which high-quality treatment or processing can be achieved without occurrences of variation in quality among heat-treated products. In this apparatus, a preheating chamber (2), six carburizing chambers (5a to 5f), a cooling chamber (6), a reheating chamber (7), a quenching chamber (8) are placed around an intermediate chamber (4) having a transport device (12). Further, a high-frequency induction heating technique is used for heating each of major ones of the treatment or processing chambers. Moreover, a plasma carburizing method is performed in the carburizing chambers.