The author calculated the freezing degree days (<0°C) in Hokkaido by use of the normals of daily mean temperature. Then, he drew the isopleth of freezing degree days of every period, of ten days and of a year. As the normals are not satisfactory by themselves for the purpose of making various civil plans, he showed the fluctuation in the freezing degree days of every period of ten days and every year, their standard deviations, and the maximum and minimum at Sapporo, Obihiro, and Wakkanai in the last 30 years.