A beam position monitoring apparatus comprising: an error operating apparatus adapted to obtain a deviation between a target position in a beam irradiation subject which is irradiated with a charged particle beam from an irradiation nozzle and an actual irradiation position which is irradiated with said charged particle beam in said beam irradiation subject in correspondence to said target position, said actual irradiation position being measured by a beam position monitor installed in said irradiation nozzle, characterized in that the apparatus is further adapted to obtain individually a systematic error and a random error for said actual irradiation position based on said deviation; and the beam position monitoring apparatus further comprising an error determination apparatus adapted to execute a first determination for determining whether said systematic error exists within a first permissible range of said systematic error and a second determination for determining whether said random error exists within a second permissible range of said random error; wherein the systematic error is obtained based on a sum of said deviations for multiple irradiation positions according to formulas and, and the random error is obtained based on a difference of said actual irradiation position and the systematic error according to formulas, and, when said target position in a certain layer among plurality of layers formed in a direction of a central axis of said irradiation nozzle is P j (j=1,...,n), said actual irradiation position corresponding to said target position is Pa j, said deviation is D j, said systematic error is Es j, said random error is Er j, and D J = P J − Pa j Es j = ∑ k = 1 j D k j Er j = P j − P a j − ∑ k = 1 j D k j an error operating apparatus adapted to obtain a deviation between a target position in a beam irradiation subject which is irradiated with a charged particle beam from an irradiation nozzle and an actual irradiation position which is irradiated with said charged particle beam in said beam irradiation subject in correspondence to said target position, said actual irradiation position being measured by a beam position monitor installed in said irradiation nozzle, characterized in that the apparatus is further adapted to obtain individually a systematic error and a random error for said actual irradiation position based on said deviation; and the beam position monitoring apparatus further comprising an error determination apparatus adapted to execute a first determination for determining whether said systematic error exists within a first permissible range of said systematic error and a second determination for determining whether said random error exists within a second permissible range of said random error; wherein the systematic error is obtained based on a sum of said deviations for multiple irradiation positions according to formulas and, and the random error is obtained based on a difference of said actual irradiation position and the systematic error according to formulas, and, when said target position in a certain layer among plurality of layers formed in a direction of a central axis of said irradiation nozzle is P j (j=1,...,n), said actual irradiation position corresponding to said target position is Pa j, said deviation is D j, said systematic error is Es j, said random error is Er j, and D J = P J − Pa j Es j = ∑ k = 1 j D k j Er j = P j − P a j − ∑ k = 1 j D k j