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. <EOS>