<p>A radiation detector which is to be inserted into a vessel so as to detect a radioactive substance accumulated in a tissue in vivo. Namely, in a detector which has a detection unit 2 provided with a bar-type scintillator 4 undergoing emission due to the incidence of radiation and by which a radioactive substance contained in a tissue in vivo is detected by transmitting the light from the scintillator via an optical fiber 10, the detection unit has such a size as allowing the insertion into a vessel and the scintillator has fine irregularity in its peripheral wall.</p>
