To provide a method and a device for changing the models of a machine tool system capable of determining an optimum timing for changing the position of a movable clamp.

A machine tool system 10 includes a first machine tool M1 and a second machine tool M2 which respectively have a machining spindles S1, S2 the positions of which can be adjusted by an adjusting mechanism 11, a workpiece moving device 14 for moving a workpiece, and a control unit 12. The workpiece moving device includes a turn table 90, a plurality of clamp units CL1, CL2, CL3 that clamp and position the workpiece, and a main motor 92 for intermittently rotating the turn table in such a manner as to position the clamp units at a workpiece loading/unloading position P0, a first machining position P1, and a second machining position P2, respectively. The control unit 12 makes the machining spindle perform the changing of the clamp positions of the clamp units by adjusting the position of the machining spindle of the first machine tool by the adjusting mechanism.