A radiation ray (gamma ray) radiated from a test subject previously dosed with radioisotope is incident on a scintillator and converted into scintillation light. A plurality of photomultipliers arranged in a hexagon receive and convert the scintillation light into electrical signals. Photomultipliers arranged in parallel with sides of the hexagon constitute a group. Output signals of the photomultipliers are supplied to a first position detector for detecting a group of photomultipliers which receives the scintillation light and deriving position information Xo and Yo representing the approximate gamma ray incident position. The position information Xo and Yo and the output signals of the photomultiplier are supplied to a second position detector for deriving position information x and y representing the precise gamma ray incident position. Outputs of the photomultiplier are supplied to a pulse height analyzer which derives incident radiation energy information z.