An abdomen width determiner determines an abdomen width value of a human subject. A memory stores a correlation between abdomen width values and waist circumferences of human beings. A waist circumference calculator calculates a waist circumference of the human subject on the basis of the abdomen width value determined by the abdomen width determiner and the correlation stored in the memory. The correlation stored in the memory may be expressed by the following regression formula: Y=aX+b where "Y" is a waist circumference of a human being, "X" is an abdomen width value of a human being, and "a" and "b" are constants.