The deterioration of concrete structures due to age, particularly in marine environments, has recently become a subject of great concern. In this study, the properties of 60-year old concrete in a marine environment are examined, taking the opportunity of the demolition of the northern breakwater of Muroran Port in 1984 to obtain samples. As a result of the study it was found that the concrete, which was made from blast furnace slag and volcanic ash and appeared to contain sea sand, had scarcely deteriorated at all, even though it had been exposed to sea water environment for 60 years.