PURPOSE:To prevent the generation of a working failure by distributing a normal vector of a three-dimensional plane in the axial directions of a fundamental coordinate system, and processing a rotating direction command of a three-dimensional circular arc interpolating command. CONSTITUTION:Each fundamental shaft data SD obtained by distributing and processing a normal vector of a three-dimensional plane derived by a plane designating G code SC from a plane designating G code storage part 8 in a circular arc command deciding/processing part 3, in the axial directions of a fundamental coordinate system is generated, and sent out to a three-dimensional circular arc projection plane determining part 4. By the three- dimensional circular arc projection plane determining part 4, a plane made by two axes of the fundamental coordinate system for projecting a three-dimensional circular arc in accordance with whether each fundamental shaft data SD is large or small is determined, and projection plane information SE is sent out to a three-dimensional circular arc rotating direction determining part 5. Subsequently, by the three-dimensional circular arc rotating direction determining part 5, the rotating direction command SF of a three-dimensional circular arc interpolating command from the circular arc command deciding/processing part 3 is converted and processed to the circular arc rotating direction command in a projection plane, based on the projection plane information SE, and this circular arc rotating direction command is sent out as a three-dimensional circular arc rotating direction command SG to a function generating part 6. In such a way, a desired three-dimensional circular arc rotating direction is obtained.
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