A thin radiation detector with a high sensitivity is described. The radiation detector has light receiving elements receiving lights emitted by scintillators, performs a photoelectric conversion by using an avalanche multiplication film formed by amorphous selenium, and reads signals by using electron beams constantly discharged from a plurality of electron beam emitting sources called as a field emission array. The avalanche multiplication film formed by amorphous selenium is quite thin and has a simple structure, so it can be formed compactly and realized at a low cost. In addition, a signal amplification degree is approximately 1000 times, so an expensive low noise amplifier or a dedicated temperature adjusting mechanism is not required, and a quantum efficiency is sufficient for a wavelength of 300˜400 nm.