The present invention acquires the number of days of a menstrual cycle and of a high temperature phase during the menstrual cycle of multiple women (S1). On the basis of whether or not the number of days of the menstrual cycle of the women is within a predetermined range and whether or not the number of days of the high temperature phase of the women is within a predetermined range, the women are categorized into a specific type in which the number of days of the menstrual cycle is unstable but the number of days of the high temperature phase is stable, and one or more types other than the specific type (S2). When categorized into the specific type, the expected date of the start of menstruation is calculated by adding the number of days based on the number of days of the high temperature phase to the woman's last ovulation day (S6). On the other hand, when categorized into a type other than the specific type, the expected date of the start of menstruation is calculated on the basis of the date of the start of the last menstruation and the number of days of the menstrual cycle of the woman (S4).