Abstract: A numerically controlled machine tool system having a tool transfer device which, with a change in the kind of workpieces to be machined in a numerically controlled machine tool, operates to replace successively unnecessary dedicated tools in a tool magazine of the machine tool with next to be used dedicated tools supplied to the vicinity of the tool magazine. A central processor reads out from an external memory device one of the numerical control (NC) programs corresponding to the kind of a lot of workpieces to be machined and transmits the NC program to a numerical controller. In advance of the machining of a last one of the lot of workpieces, the central processor modifies the NC program by inserting respectively into correct portions of the NC program a number of replacing command data each instructing that one of the dedicated tools in the tool magazine designated by one of the replacing command data be replaced.