PURPOSE: To enable a conversion section to be eliminated, and a compact high frequency accelerator to be provided by applying the same sectional form as that of a beam duct to a beam passage section in the accelerating electrode of a high frequency accelerating cavity.

CONSTITUTION: A tuner 5 is inserted in or pulled out of a cavity with a drive device 6. When the tuner 5 is inserted, a resonance frequency increases, and this frequency decreases when the tuner is pulled out. In order to maintain ultra-high vacuum state, a gasket 8 is fitted to the joint surface of each member. A beam passage section 3a in the cavity has the same form as a beam duct 7, for example, a racing track form. As a result, the section 3a can be connected to the beam duct 7 without any need of conversion section. According to this construction, a high frequency accelerating cavity of compact and high performance type can be provided without need of any converter, while the effect of induction field on beam acceleration is reduced. In this case, the form of the duct 7 may be rectangular or elliptical as an alternative for the racing track form.