In the city of Otaru there are two historical stone remains using porous tuff. Subjected to multiple freezing-thawing cycles, the stone remains were damaged. To preserve them, on-the-spot investigations were conducted. The number of freezing-thawing cycles was estimated by the measurements of temperature fluctuations in the rock blocks or bedrocks. If rocks of the bedrocks freeze and thaw with temperatures fluctuating between the maximum of +4°C and the minimum of -4°C, the rocks tend to break up. There were six freezing-thawing cycles in this critical temperature range during the observed period in winter season.