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Structural materials employed for high-temperature and high-hydrogen-pressure service require excellent strength, and 2.25Cr-1Mo steel is widely used for such purposes. For instance, it is used in pressure vessels employed for hydro desulfurization, hydrocracking, and coal-liquefaction units. However, instances of hydrogen-related cracks occurring during a shutdown period have been reported when 2.25Cr-1Mo steel is used for long periods at high temperatures and high hydrogen pressures. As the risk of internal hydrogen embrittlement during shutdown, along with the detrimental effect of long-term temper-embrittlement, is a particular concern for the integrity and safe operation of pressure vessels, the combined effect of temper-embrittlement and hydrogen embrittlement in Cr-Mo steels was previously investigated using a step-cooling treatment, which is an accelerated temper-embrittlement treatment. In practical usage, it is important to ascertain the threshold stress-intensity factor at the onset of hydrogen-enhanced crack growth, KIH, of the material following long-term use. In the present study, we employed the offset potential-drop method to clarify the threshold stress-intensity factor, KIH for 2.25Cr-1Mo steel that had been used in actual equipment for a period of approximately 64,000 h at service temperatures (360–440 °C).