To provide a synchrocyclotron type particle beam accelerator comprising an acceleration cavity which can modulate a frequency, in which a beam fetching is easier than that of the conventional one, provide a small-sized particle beam accelerator, and provide a particle beam therapy system with the particle beam accelerator.SOLUTION: On a surface opposite to an intermediate flat surface 2 of an upper magnetic pole 8 and a lower magnetic pole 9 of a magnetic device 1, concave parts 21a, 21b, 21c, 21d and convex parts 22a, 22b, 22c, 22d are alternately arranged along a beam peripheral direction. In the convex parts 22a, 22b, 22c, 22d, each angle width θ of the convex parts 22a, 22b, 22c, 22d when viewing it from a center O1 of a beam peripheral locus is narrowed in accordance with an increase of a beam energy, and in an outer peripheral region of the upper magnetic pole 8 and the lower magnetic pole 9 of the concave part 21a, an inlet of an emission channel 1019 that fetches a beam accelerated to a prescribed energy to the outside of an accelerator 1004 is provided.SELECTED DRAWING: Figure 4【課題】 周波数変調が可能な加速空洞を備えたシンクロサイクロトロン型の粒子線加速器において、ビーム取出しが従来に比べて容易であり、かつ小型の粒子線加速器とそれを備えた粒子線治療システムを提供する。【解決手段】 磁石装置１のうち、上部磁極８，下部磁極９の中間平面２に対向する面には、ビーム周回方向に沿って凹部２１ａ，２１ｂ，２１ｃ，２１ｄと凸部２２ａ，２２ｂ，２２ｃ，２２ｄとが交互に配置されており、凸部２２ａ，２２ｂ，２２ｃ，２２ｄは、ビーム周回軌道の中心Ｏ１から見たときの凸部２２ａ，２２ｂ，２２ｃ，２２ｄの角度幅θがビームエネルギーの増大に従って狭まっていき、凹部２１ａの上部磁極８，下部磁極９外周領域に、所定エネルギーに加速されたビームを加速器１００４の外に取り出す出射チャネル１０１９の入口が設けられている。【選択図】 図４
