An adaptive control system for adaptively controlling a turning operation performed at a work piece by a turning tool adjusts a controlled input operation parameter F to maintain an output operation parameter .DELTA.M substantially at a predetermined value .DELTA.M.sub.o to compensate variation of the output operation paramemeter .DELTA.M caused by the variation of at least one operation condition B=B (t). The system comprises a sensor (8) of the output operation parameter .DELTA.M for providing a signal U.sub.c proportional to a current value .DELTA.M.sub.c and an adaptive controller (10) for determining a value F.sub.c to which the input operation parameter F should be adjusted, as a function of kU.sub.c, where k is a signal transmission coefficient which comprises an invariant signal transmission coefficient component k.sub.o inversely proportional to .DELTA.M.sub.o. The adaptive controller includes an amplifier transforming the signal U.sub.c into k.sub.o U.sub.c, and an input parameter override unit controlled to adjust the controlled operation input parameter to F.sub.c. The adaptive controller further comprises a correction processing means calculating k.sub.c U.sub.c, where k.sub.c is a varying signal transmission coefficient component whose current values depend on the variation of the operation condition B=B (t). The adaptive controller is capable of calculating k=f (k.sub.o, k.sub.c).