A particle beam scanning irradiation system comprising: - a computer that is adapted to calculate a planned irradiating particle count (Ni) of a particle beam for each of irradiation spots (i), on the basis of a relative irradiating particle count (Ri) and a prescription dose (DO) that are determined from a particle-beam therapy plan; and - a particle beam irradiation device that is adapted to irradiate a diseased portion with the particle beam, wherein the computer is adapted to simulate an irradiation process of the particle beam at each irradiation spot (i), on the basis of the planned irradiating particle count (Ni) and a beam current waveform (I(t)) of the particle beam, and is adapted to execute - a first step of calculating an irradiating particle count ( Δ Ni) of the particle beam irradiating the diseased portion during a scan shift of the particle beam, wherein the particle beam is shifted between adjoining spots during the scan shift; - a second step of correcting the planned irradiating particle count (Ni) for each irradiation spot by using the irradiating particle count (ΔNi) during the scan shift in order to obtain a corrected planned irradiating particle count (Ni_corrected); and - a third step of converting the corrected planned-irradiating particle count (Ni_corrected) into a count value used in a dose monitor, and the particle beam irradiation device is adapted to irradiate the diseased portion with the particle beam, on the basis of the count value obtained in the third step , wherein the second step includes calculations of subtracting half the irradiating particle count during the scan shift from a planned irradiating particle count for an irradiation spot before the scan shift and from a planned irradiating particle count for an irradiation spot after the scan shift. - a computer that is adapted to calculate a planned irradiating particle count (Ni) of a particle beam for each of irradiation spots (i), on the basis of a relative irradiating particle count (Ri) and a prescription dose (DO) that are determined from a particle-beam therapy plan; and - a particle beam irradiation device that is adapted to irradiate a diseased portion with the particle beam, wherein the computer is adapted to simulate an irradiation process of the particle beam at each irradiation spot (i), on the basis of the planned irradiating particle count (Ni) and a beam current waveform (I(t)) of the particle beam, and is adapted to execute - a first step of calculating an irradiating particle count ( Δ Ni) of the particle beam irradiating the diseased portion during a scan shift of the particle beam, wherein the particle beam is shifted between adjoining spots during the scan shift; - a second step of correcting the planned irradiating particle count (Ni) for each irradiation spot by using the irradiating particle count (ΔNi) during the scan shift in order to obtain a corrected planned irradiating particle count (Ni_corrected); and - a third step of converting the corrected planned-irradiating particle count (Ni_corrected) into a count value used in a dose monitor, and the particle beam irradiation device is adapted to irradiate the diseased portion with the particle beam, on the basis of the count value obtained in the third step , wherein the second step includes calculations of subtracting half the irradiating particle count during the scan shift from a planned irradiating particle count for an irradiation spot before the scan shift and from a planned irradiating particle count for an irradiation spot after the scan shift. <EOS>