Structural Steels with higher strength are recently required. Therefore, environmental degradation by hydrogen, delayed fracture, should be afraid. Absorbed hydrogen contents in structural steels have been investigated by thermal hydrogen analysis. Hydrogen entry into steel has been directly measured by electrochemical hydrogen permeation technique. Hydrogen entry into steel occurs immediately after weathering by atmospheric corrosion. The maximum absorbed hydrogen content is observed at half a year. Hydrogen entry into steel is not affected by area location, weathering conditions, air born salts and so on. Hydrogen entry is due to the pH drop of hydrolysis of Fe²⁺ or Fe³⁺ with initial rust formation.