A radiotherapy system includes: a waveguide, an adjustable waveguide, and a non-reciprocal circuit element. The waveguide transmits a high-frequency wave from a high-frequency power source to an acceleration tube. The adjustable waveguide is included in said waveguide and transforms a part of said waveguide. The non-reciprocal circuit element is provided between said acceleration tube and said adjustable waveguide in said waveguide. Said acceleration tube accelerates charged particles for generating therapeutic radiation by using said high-frequency wave.