A radiation detector module includes a radiation detecting substrate including a plurality of semiconductor devices mounted thereon for detecting radiation, a fixing member for holding the radiation detecting substrate, and a circuit substrate connected to the radiation detecting substrate. The radiation detecting substrate further includes a mounting board including an edge portion connected to the circuit substrate at one end thereof, and a flexible substrate including a wiring pattern connected to the plurality of semiconductor devices. The fixing member includes a bottom, a first side wall extending in a normal direction to the bottom from one end of the bottom, a second side wall extending in the normal direction to the bottom from the other end of the bottom, and substrate supporting portions on the first side wall and the second side wall.