A testing apparatus, which can simultaneously monitor the hydrogen gas release and the progress of fracture, was newly developed in order to clarify the behavior of hydrogen atoms on hydrogen embrittlement in aluminum alloys. This apparatus is composed of a tensile testing machine equipped with two mass spectrometers and a high-speed microscope placed in an ultrahigh vacuum (UHV) chamber. By using this apparatus, we have succeeded in detecting hydrogen (deuterium) gas evolution dynamically during grain boundary fracture in a 7075 aluminum alloy indicating hydrogen embrittlement. Concentration of hydrogen atoms at the grain boundary associated with hydrogen embrittlement was estimated to be more than 3.0×10⁻⁷ mol/m². It was also clarified that deuterium atoms introduced from the testing atmosphere were released not only at the moment of fracture but also at the plastic deformation.