A self-powered gamma detector which can hold the influence of neutron rays to the minimum, and can measure the dose of gamma rays accurately under a high neutron environment in the reactor is disclosed. The self-powered gamma detector has a columnar emitter member; a collector member arranged in surroundings of the emitter member through an insulating member, and the strength of gamma rays is measured by detecting the value of a current which flows between the emitter member and an MI cable connected with the emitter member, and the collector member. Especially, the emitter member consists of pure tungsten (W), whose inevitable impurities is in 0.03 percent by weight or less.