Hydrogen permeation behavior in carbon steel exposed to gaseous hydrogen was visualized using a hydrogen microprint technique (HMT). Effects of hydrogen gas pressure and charging time on the hydrogen permeation were particularly examined. The amount of permeated hydrogen was dependent on the charging time during the exposure to gaseous hydrogen. It was found that silver particles, which represented the evolution site of hydrogen atoms, were distributed almost uniformly in the matrix after hydrogen gas charging. These particles were arranged at the periphery of the second phase particles such as Al₂O₃. Area density of the silver particles clearly increased when the time for hydrogen gas charging was increased. Preferential accumulation of silver particles around Al₂O₃ particles was clearly identified; however, no silver particles were observed directly on the Al₂O₃ particles. This indicated that hydrogen atoms were diffused not through the inside of the second phase particles but through the interface between the second phase particles and the matrix phase.