A charged particle beam irradiation system comprising: a charged particle beam generating unit configured to accelerate and extract a charged particle beam; a scanning electromagnet configured to scan the accelerated charged particle beam; a beam irradiation apparatus configured to irradiate the accelerated charged particle beam to a plurality of irradiation spots respectively set for a plurality of layers which is formed by dividing an irradiation target in a traveling direction of the charged particle beam; wherein a part of or all of the plurality of the irradiation spots is divided into a plurality of splits; the splits being created by division of the dose of an irradiation spot into the plurality of beam irradiation sections; a beam radiation dose measuring instrument configured to obtain a dose of the charged particle beam for passing through the beam irradiation apparatus; and a beam position measuring instrument configured to obtain one or both of the position and the width of the charged particle beam scanned by the scanning electromagnet during irradiation to the irradiation spot with the charged particle beam with respect to a split; characterised in that: the beam position measuring instrument is adapted to start calculation of one or both of the position and the width of the charged particle beam for each irradiation spot when a split dose satisfied signal is issued, to obtain a result of the calculation, and to determine whether the obtained result is within an allowable range, and and after the determination for a split has finished, to start calculation of one or both of the position and the width of the charged particle beam obtained for the next split. a charged particle beam generating unit configured to accelerate and extract a charged particle beam; a scanning electromagnet configured to scan the accelerated charged particle beam; a beam irradiation apparatus configured to irradiate the accelerated charged particle beam to a plurality of irradiation spots respectively set for a plurality of layers which is formed by dividing an irradiation target in a traveling direction of the charged particle beam; wherein a part of or all of the plurality of the irradiation spots is divided into a plurality of splits; the splits being created by division of the dose of an irradiation spot into the plurality of beam irradiation sections; a beam radiation dose measuring instrument configured to obtain a dose of the charged particle beam for passing through the beam irradiation apparatus; and a beam position measuring instrument configured to obtain one or both of the position and the width of the charged particle beam scanned by the scanning electromagnet during irradiation to the irradiation spot with the charged particle beam with respect to a split; characterised in that: the beam position measuring instrument is adapted to start calculation of one or both of the position and the width of the charged particle beam for each irradiation spot when a split dose satisfied signal is issued, to obtain a result of the calculation, and to determine whether the obtained result is within an allowable range, and and after the determination for a split has finished, to start calculation of one or both of the position and the width of the charged particle beam obtained for the next split. the beam position measuring instrument is adapted to start calculation of one or both of the position and the width of the charged particle beam for each irradiation spot when a split dose satisfied signal is issued, to obtain a result of the calculation, and to determine whether the obtained result is within an allowable range, and and after the determination for a split has finished, to start calculation of one or both of the position and the width of the charged particle beam obtained for the next split.