A radiation detector configured to detect radiation in a first energy range and radiation in a second energy range entered from a front side, comprising: a first scintillator layer extending along a predetermined direction, for converting radiation in the first energy range to light; a first photodetector having a plurality of first photo-detecting sections being disposed one-dimensionally along the predetermined direction and fixed to a front side of the first scintillator layer, for converting light converted by the first scintillator layer to an electrical signal, and a first substrate with the first photo-detecting sections provided; a second scintillator layer extending in the predetermined direction and being brought in contact with a rear side of the first scintillator layer, for converting radiation in the second energy range to light; a second photodetector having a plurality of second photo-detecting sections being disposed one-dimensionally along the predetermined direction and fixed to a rear side of the second scintillator layer, for converting light converted by the second scintillator layer to an electrical signal, and a second substrate with the second photo-detecting sections provided, characterized in that: the first scintillator layer has a thickness thinner than that of the second scintillator layer, when viewed from a front side, in a direction perpendicular to the predetermined direction, one edge portion of the first substrate is located outside further than one edge portion of the second substrate, and the other edge portion of the second substrate is located outside further than the other edge portion of the first substrate; the radiation detector further comprising a slit structure being disposed on a front side of the first photodetector, for passing radiation in the first energy range and radiation in the second energy range, wherein the slit structure has a first plate-like member formed with a slit extending in the predetermined direction, and a second plate-like member that supports the first plate-like member from a rear side, and in the second plate-like member, a wall portion provided in a standing condition toward the rear side is formed along one edge portion and the other edge portion of the slit. a first scintillator layer extending along a predetermined direction, for converting radiation in the first energy range to light; a first photodetector having a plurality of first photo-detecting sections being disposed one-dimensionally along the predetermined direction and fixed to a front side of the first scintillator layer, for converting light converted by the first scintillator layer to an electrical signal, and a first substrate with the first photo-detecting sections provided; a second scintillator layer extending in the predetermined direction and being brought in contact with a rear side of the first scintillator layer, for converting radiation in the second energy range to light; a second photodetector having a plurality of second photo-detecting sections being disposed one-dimensionally along the predetermined direction and fixed to a rear side of the second scintillator layer, for converting light converted by the second scintillator layer to an electrical signal, and a second substrate with the second photo-detecting sections provided, characterized in that: the first scintillator layer has a thickness thinner than that of the second scintillator layer, when viewed from a front side, in a direction perpendicular to the predetermined direction, one edge portion of the first substrate is located outside further than one edge portion of the second substrate, and the other edge portion of the second substrate is located outside further than the other edge portion of the first substrate; the radiation detector further comprising a slit structure being disposed on a front side of the first photodetector, for passing radiation in the first energy range and radiation in the second energy range, wherein the slit structure has a first plate-like member formed with a slit extending in the predetermined direction, and a second plate-like member that supports the first plate-like member from a rear side, and in the second plate-like member, a wall portion provided in a standing condition toward the rear side is formed along one edge portion and the other edge portion of the slit. the first scintillator layer has a thickness thinner than that of the second scintillator layer, when viewed from a front side, in a direction perpendicular to the predetermined direction, one edge portion of the first substrate is located outside further than one edge portion of the second substrate, and the other edge portion of the second substrate is located outside further than the other edge portion of the first substrate; the radiation detector further comprising a slit structure being disposed on a front side of the first photodetector, for passing radiation in the first energy range and radiation in the second energy range, wherein the slit structure has a first plate-like member formed with a slit extending in the predetermined direction, and a second plate-like member that supports the first plate-like member from a rear side, and in the second plate-like member, a wall portion provided in a standing condition toward the rear side is formed along one edge portion and the other edge portion of the slit.