A beam transport system for transporting a charged particle beam emitted from an accelerator to an irradiation position (terminal point T), comprising - at least one bending electromagnet that is adapted to deflect the charged particle beam; - at least two quadrupole electromagnets that are adapted to focus or defocus the charged particle beam; - at least one beam profile monitor that is adapted to detect profile data of the charged particle beam; - a beam analyzer that is adapted to calculate based on the profile data, a beam temporal-variation related amount that is a beam-position temporal variation amount or a beam diameter, at the beam profile monitor; - an optical parameter calculator that is adapted to calculate optical parameters of the beam transport system; and - an electromagnet power source that is adapted to set an excitation current of each of the bending electromagnet and the quadrupole electromagnets, based on the optical parameters calculated by the optical parameter calculator; characterised in that the optical parameter calculator is adapted to: - - calculate based on the beam temporal-variation related amount, a start-point momentum dispersion function that is a momentum dispersion function of the charged particle beam at a start point in design (start point S) of the beam transport system that is set on a beam trajectory of the accelerator; and - - calculate the optical parameters using, as an initial condition, the start-point momentum dispersion function and a beginning condition at the irradiation position (terminal point T) at the time of detecting the profile data. - at least one bending electromagnet that is adapted to deflect the charged particle beam; - at least two quadrupole electromagnets that are adapted to focus or defocus the charged particle beam; - at least one beam profile monitor that is adapted to detect profile data of the charged particle beam; - a beam analyzer that is adapted to calculate based on the profile data, a beam temporal-variation related amount that is a beam-position temporal variation amount or a beam diameter, at the beam profile monitor; - an optical parameter calculator that is adapted to calculate optical parameters of the beam transport system; and - an electromagnet power source that is adapted to set an excitation current of each of the bending electromagnet and the quadrupole electromagnets, based on the optical parameters calculated by the optical parameter calculator; characterised in that the optical parameter calculator is adapted to: - - calculate based on the beam temporal-variation related amount, a start-point momentum dispersion function that is a momentum dispersion function of the charged particle beam at a start point in design (start point S) of the beam transport system that is set on a beam trajectory of the accelerator; and - - calculate the optical parameters using, as an initial condition, the start-point momentum dispersion function and a beginning condition at the irradiation position (terminal point T) at the time of detecting the profile data. - - calculate based on the beam temporal-variation related amount, a start-point momentum dispersion function that is a momentum dispersion function of the charged particle beam at a start point in design (start point S) of the beam transport system that is set on a beam trajectory of the accelerator; and - - calculate the optical parameters using, as an initial condition, the start-point momentum dispersion function and a beginning condition at the irradiation position (terminal point T) at the time of detecting the profile data.