The object is to provide a beam transport system that can nearly nullify a chromatic aberration of a beam at an irradiation position of the beam even when it is a beam emitted from a small-sized synchrotron. According to a beam transport system (4) of the invention, it is characterized in that: based on a beam temporal-variation related amount that has been calculated by a beam analyzer (14) and that is a beam-position temporal variation amount or a beam diameter at a beam profile monitor (11), an optical parameter calculator (17) calculates a start- point momentum dispersion function that is a momentum dispersion function (·, ·') of a charged particle beam (31) at a start point (S) in design of the beam transport system (4) that is set on a beam trajectory of the accelerator (3); and calculates optical parameters using, as an initial condition, the start-point momentum dispersion function and a beginning condition at an irradiation position (T) at the time of detecting profile data.