This multileaf collimator (60) is provided with multiple leaves (70) which limit the area irradiated by a radiation beam (S2) and which are arranged in the thickness direction. Each of the leaves (70) is provided with a convex portion (75) which is formed protruding towards a first surface (70f) of the leaf (70), a groove (76) which is formed in the second surface (70g) of the leaf (70) and into which the convex portion (75) formed on another leaf (70) is inserted, a first curved surface (220A) which is formed between the first surface (70f) of the leaf (70) and the lateral surface (75a) of the convex portion (75), and a second curved surface (220B) which is formed between the second surface (70g) of the leaf (70) and the lateral surface (76a) of the groove (76).
