The present invention provides a radiation detector capable of detecting the radioactive substance accumulated in the tissue inside a body by inserting a detection unit into blood vessels. In the radiation detector of the present invention, comprising a detection unit (2) having a bar-type scintillator (4) which emits light by an incidence of radiation so as to transmit the light from the scintillator through an optical fiber, the detection unit is formed in a size capable of being inserted into the tubules while fine convexoconcaves are provided on the peripheral surface of the scintillator.