This paper is a report on the investigation results of the Former Shime Mining Office Vertical Derrick, which was built in 1943 and was 75 years old at the time of the investigation. The building suffered from serious deterioration including rebar corrosion, which led to large area of concrete spalling. In this report, aggregation data of concrete spalling has been presented along with the investigation results of cover thickness. According to the investigation results, concrete spalling occurred in places where cover thickness was less than 25 mm. And According to the aggregation data, concrete spalling flakes increased by approximately 200 pieces per year, with the average size of 10 cm in diameter. Also, a possible relation between concrete spalling and rainfall was observed during the investigation. Equations using the proportion of wet area affected by rainfall to estimate the area of spalling are proposed, although the value of empirical coefficientaneeds further discussion, including collecting data from other buildings where spalling occurs.