A scanning magnet of an irradiation nozzle is controlled to irradiate the ion beam irradiated from a synchrotron accelerator to a target position Pi,j of a spoti,j in a certain layer Li of a target volume, using a scanning control apparatus. A deviation Dj between the target position Pi,j and an actual irradiation position Pai,j is obtained. Using the deviation Dj, a systematic error Esi,j and a random error Eri,j of the actual irradiation position Pai,j are obtained. Whether the systematic error Esi,j exists within a first permissible range of the systematic error Esi,j is determined. Whether the random error Eri,j exists within a second permissible range of the random error Eri,j is determined. When the systematic error Esi,j or the random error Eri,j is deviated from the permissible range, the irradiation of the ion beam to the target volume is stopped.
