A number of studies have been conducted to investigate fracture toughness of structural steels. However, there are a limited number of studies for long-term used structural steels. Given the recent engineering practice of life extension for the existent steel infrastructures, it is very important to develop reliable methodologies for proper evaluation of infrastructure integrity. Correspondingly, as an extension of previous study, fracture toughness of a carbon steel (JIS SS400) that has been used for the penstock of a hydroelectric power plant for about 50 years was examined. By measuring the critical crack tip opening displacement (CTOD) in conjunction with analysis for chemical compositions, the characteristics of fracture toughness in terms of critical CTOD was investigated.