Pulse waves of a subject are detected in time sequence. Amplitude of the pulse waves is detected, and an interval between two pulse waves adjacent along a time axis is detected. A first change ratio of the interval along the time axis, and a second change ratio of the amplitude divided by the interval along the time axis are calculated respectively. By comparing the first change ratio and the second change ratio with a first threshold and a second threshold respectively, it is decided whether the pulse waves of the subject are irregular.
