A high-energy ray detector includes a detection unit in a vacuum container. The detection unit includes a first electron multiplier, a second electron multiplier, and an electron collector. Each of the first electron multiplier and the second electron multiplier has one or more MCPs each configured to emit electrons by interaction with an incident high-energy ray (γ-ray, X-ray (in particular hard X-ray), or neutron ray), and multiply and output the electrons. The electron collector is transmissive for the high-energy ray. The electron collector is configured to collect the electrons multiplied and output from each of the first electron multiplier and the second electron multiplier, and output an electric pulse signal.