Simulated are postures each formed by a manipulator arm and a surgical instrument when a tip end portion of the surgical instrument is located at a three-dimensional position of each lattice point in a lattice model set so as to correspond to the manipulator arm, the surgical instrument being held by the manipulator arm. Extracted is such a combination of the lattice points in the lattice models that the postures interfere with each other. Data of the combination which causes the interference is stored in a storage portion. It is determined whether or not an operating command transferred to a control portion corresponds to the combination which causes the interference. When it is determined that the operating command corresponds to the combination which causes the interference, the control portion executes interference preventing processing.