A sphere size measuring unit (15) measures respective diameters of an object sphere in the course of processing and a standard sphere made of the same material as that of the object sphere and furthermore having a target diameter of the object sphere. A size difference calculation unit (161) calculates a size difference between the diameter of the object sphere and the diameter of the standard sphere. A determination unit (162) compares the obtained size difference with a threshold value so as to determine whether or not the diameter of the object sphere reaches the target value.