Provided is a radiation detector, including: a two-dimensional light receiving element including a plurality of pixels; and a scintillator layer having multiple scintillator crystals two-dimensionally arranged on a light receiving surface of the two-dimensional light receiving element, in which: the scintillator crystal includes two crystal phases, which are a first crystal phase including a material including a plurality of columnar crystals extending in a direction perpendicular to the light receiving surface of the two-dimensional light receiving element and having a refractive index n1, and a second crystal phase including a material existing between the plurality of columnar crystals and having a refractive index n2; and a material having a refractive index n3 is placed between adjacent scintillator crystals, the refractive index n3 satisfying a relationship of one of n1≦n3≦n2 and n2≦n3≦n1.