A radio frequency coupler includes a waveguide having an outer conductor and an inner conductor in a coaxial tube shape. The waveguide extends linearly from a power supply side, is bent in an L shape at a bend section, and extends linearly toward an acceleration cavity. A refrigerant passage part passes through the outer conductor and the inner conductor at the bend section from the outside of the waveguide toward the acceleration cavity and is connected to the interior of the inner conductor. The refrigerant passage part has a passage tube through which refrigerant passes between the inner part of the tip on the acceleration cavity side of the inner conductor, and the outer part of the waveguide, and the portion of the passage tube that is exposed to a radio frequency transmission space formed between the outer conductor and the inner conductor being formed of an insulator.