From pulse wave data obtained from change in a blood flow, an average interval of a prescribed number of immediately preceding pulses or pulses in an immediately preceding prescribed time period is calculated. Next, an average-calculation range is determined on the basis of at least one of an amount of fluctuation and an evaluation result. The amount of fluctuation is a value calculated on the basis of a difference between the average interval and an interval of a pulse wave newly detected from the pulse wave data. The evaluation result is a result of evaluating the interval of the newly detected pulse wave by at least one evaluation factor. Thereafter, an average pulse wave interval value is calculated by averaging pulse wave intervals on the basis of the average-calculation range, and a pulse rate is calculated on the basis of the average pulse wave interval value.