The refurbishment of JMTR is scheduled from the beginning of FY 2007 to the end of FY 2010. An integrity investigation for concrete structures of the JMTR buildings was carried out in early phase of the refurbishment. These concrete structures have been used for more than 40 years after completion of the construction and are expected to be used more than 20 year from restart of the JMTR. The investigated items were the concrete surface deterioration, the rebound number (nondestructive strength test), the comprehensive strength using drilled concrete core test piece, the carbonation depth, the reinforced bar corrosion and the chloride ion content. This paper describes the investigated results and repair work of the vent stack, the trenches, the canal building and the filter bank.