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Despite of numerous researches on hydrogen embrittlement in high strength steels, the hydrogen embrittlement mechanism [1-3] is still unclear. This reason is that it is uncertain how hydrogen trapping state in steel is related to the strength degradation by hydrogen embrittlement. Hydrogen trapping state in steel has been quantitatively analyzed by thermal desorption spectrometry [4, 5]. This method is being applied to the research on hydrogen embrittlement [6]. In steel, hydrogen is trapped at the lattice defects such as vacancies, dislocations and second phase particles (precipitates and inclusions). Furthermore, hydrogen trapped at above trap sites is classified into two kinds of states which are respectively called diffusible and irreversible hydrogen trap sites [4]. Diffusible hydrogen can diffuse at room temperature and contributes to hydrogen embrittlement under static loading.