A radiation detector that improves accurately a fluorescence emission-time. A limiter circuit instead of a low-pass filter and a high-pass filter removes a noise component of the amplifier output. The limiter circuit blocks passing through the amplification signal when the amplification signal output from the amplifier a is lower than the limit level. Accordingly, a noise component output not related to the fluorescence detection from the amplifier a is blocked by the limiter circuit L and is unable to reach to the addition circuit. When the amplification signal output from the amplifier a is larger than the limit level, the limiter circuit L passes through such amplification signal; so that the signal, which is related to a fluorescence detection, that the amplifier a outputs can be absolutely input into the fluorescence emission-time calculation element.