A radiation detector includes a semiconductor element capable of detecting a radiation, a substrate on which the semiconductor element is mounted, and a support member disposed adjacent to the semiconductor element for supporting the substrate. The substrate includes a first end portion on which the semiconductor element is mounted, and a second end portion opposite the first end portion for disposing the support member. The support member includes a first support and a second support, and the first support and the second support each includes a protrusion and an engagement hole for engaging with the protrusion. The protrusion of the first support engages with the engagement hole of the second support to fix the first support and the second support such that the substrate is compressed and supported by the first support and the second support.