A pulse wave period measuring apparatus according to an embodiment of the present invention detects a pulse wave of a subject, detects a maximum value and a minimum value in a predetermined period of the pulse wave, calculates one or more reference values between the maximum value and the minimum value in the predetermined period, based on one or more internally dividing ratios common to plural predetermined periods of the pulse wave, calculates two or more times in the predetermined period, the apparatus calculating the two or more times at which the pulse wave intersects with the one or more reference values, calculates a time period between each calculated time in the predetermined period and a corresponding calculated time in a last period before the predetermined period, as a pulse wave period of the pulse wave, calculates a variation of pulse wave periods, for two or more pulse wave periods calculated between the predetermined period and the last period before the predetermined period, and calculates, based on the variation of the pulse wave periods, the stability of measuring the pulse wave periods.
