An interference judgment method of a machine tool for judging a presence or absence of an interference between components of a machine tool when operating the machine tool according to a machining program, comprising: a setting step of setting a workpiece origin providing a reference point of a workpiece, a workpiece coordinate system based on the workpiece origin, a machine tool model obtained by combining shape models of the components, the machine tool model including a workpiece model corresponding to the workpiece, coordinate data of a workpiece model origin in a machine coordinate system, the workpiece model origin providing a reference point of the workpiece model and corresponding to the workpiece origin, and a reference surface (M1x) of the workpiece model which corresponds to a reference surface (Wx) of the workpiece and which is parallel with an X-axis or a Y-axis of the machine coordinate system; a measurement step of measuring the workpiece mounted to a workpiece mounting part to determine an origin positional deviation amount corresponding to the coordinate data of the workpiece origin in the machine coordinate system, and an angular deviation amount (θ) corresponding to an angle of the reference surface (Wx) of the workpiece in the machine coordinate system in a C-axis direction, which is the direction around which the workpiece rotates together with a rotary table; a storing step of storing the origin positional deviation amount and the angular deviation amount (θ) in a workpiece origin offset storage part provided in an NC unit which controls the operation of the machine tool; a correcting step of reading the origin positional deviation amount and the angular deviation amount (θ) from the workpiece origin offset storage part, when a positioning signal set in a machining program in advance by an M code is output when said M code is read, to correct the machine tool model set at the setting step by moving the workpiece model in parallel by a positional deviation amount between the origin positional deviation amount and the coordinate data of the workpiece model origin, and by rotating the workpiece model about the C-axis direction by the angular deviation amount; and a judgment step of judging the presence or absence of the interference between the components based on the machine tool model corrected at the correcting step. a setting step of setting a workpiece origin providing a reference point of a workpiece, a workpiece coordinate system based on the workpiece origin, a machine tool model obtained by combining shape models of the components, the machine tool model including a workpiece model corresponding to the workpiece, coordinate data of a workpiece model origin in a machine coordinate system, the workpiece model origin providing a reference point of the workpiece model and corresponding to the workpiece origin, and a reference surface (M1x) of the workpiece model which corresponds to a reference surface (Wx) of the workpiece and which is parallel with an X-axis or a Y-axis of the machine coordinate system; a measurement step of measuring the workpiece mounted to a workpiece mounting part to determine an origin positional deviation amount corresponding to the coordinate data of the workpiece origin in the machine coordinate system, and an angular deviation amount (θ) corresponding to an angle of the reference surface (Wx) of the workpiece in the machine coordinate system in a C-axis direction, which is the direction around which the workpiece rotates together with a rotary table; a storing step of storing the origin positional deviation amount and the angular deviation amount (θ) in a workpiece origin offset storage part provided in an NC unit which controls the operation of the machine tool; a correcting step of reading the origin positional deviation amount and the angular deviation amount (θ) from the workpiece origin offset storage part, when a positioning signal set in a machining program in advance by an M code is output when said M code is read, to correct the machine tool model set at the setting step by moving the workpiece model in parallel by a positional deviation amount between the origin positional deviation amount and the coordinate data of the workpiece model origin, and by rotating the workpiece model about the C-axis direction by the angular deviation amount; and a judgment step of judging the presence or absence of the interference between the components based on the machine tool model corrected at the correcting step.