PROBLEM TO BE SOLVED: To provide a particle beam treatment system which is able to sufficiently expand a Bragg peak width and realizes a small spot size capable of forming a uniform dose distribution on a target laterally extending largely without making an irradiation field forming device and a rotary gantry larger, and to provide a ridge filter and a method of manufacturing the same suitable for the particle beam treatment system and easily processable.SOLUTION: A structure 301 which makes up a ridge filter 101 is linearly symmetrical about a line passing through the center of the structure 301 in a vertical direction with respect to a depth direction. A small structure 305 obtained by dividing the structure 301 along the line of symmetry is laterally asymmetrical with respect to a centerline in a repetition direction and is point-symmetrical with respect to an intersection of the centerline in the repetition direction and the centerline in the depth direction. The most upstream face 301A and the most downstream face 301B in the depth direction are equal in the thickness in the repetition direction, and any portion having a larger thickness in the repetition direction than the most upstream face 301A and the most downstream face 301B is not present in the depth direction.SELECTED DRAWING: Figure 6【課題】ブラッグピーク幅を十分に広げることができ、且つ照射野形成装置および回転ガントリーの大型化を招くことなく横方向に大きく広がった標的に一様な線量分布を形成可能なスポットサイズの小さな粒子線治療システムと、このような粒子線治療システムに好適且つ加工の容易なリッジフィルタとその製造方法を提供する。【解決手段】リッジフィルタ１０１を構成する構造体３０１は、構造体３０１の中心を通る深さ方向と垂直な線に線対称であり、この線で構造体３０１を分割して得られる小構造体３０５は、繰り返し方向の中心線に対して左右非対称形状で、繰り返し方向の中心線と深さ方向の中心線との交点に対して点対称形状で、深さ方向の最上流面３０１Ａおよび最下流面３０１Ｂの繰り返し方向の厚さが等しく、最上流面３０１Ａおよび最下流面３０１Ｂより繰り返し方向の厚さが厚い部分が深さ方向において存在しない構造である。【選択図】 図６
