Steel workpiece oil quenching method

A steel workpiece maintained at a specified quenching temperature is rapidly cooled to a temperature just above the martensite transformation start point (Ms point) by being immersed in a high-temperature quenching oil. Thereafter, the steel workpiece is taken out of the high-temperature quenching oil so as to be soaked by the heat possessed by the steel workpiece and subsequently cooled by being immersed in the high-temperature quenching oil. Through these processes, a temperature difference between steel workpieces or the portions of a steel workpiece in the martensite transformation stage is reduced, and a cooling speed in a high-temperature region (not lower than about 550° C.) is made to be a slow speed sufficient for restraining a thermal distortion, by which the quenching distortion and quenching variation can be reduced.