Local maxima values and local minima values are derived from eyelid openness time series data in a segment in which a continuous closed eye period of extracted blinks is a specific time duration (for example 1 second) or longer. When plural local minima values are present in the segment of continuous closed eye period of 1 second or longer, blinks are extracted passing over and back through each variable closed eye threshold value of a variable closed eye threshold that is slid in a direction from the derived local maxima value towards the local minima value in set steps to a low value, and a inter-blink interval derived. Determination is made that a blink burst has occurred when the derived inter-blink interval is 1 second or less, and say greater than 0.2 seconds, thereby detecting a blink burst. Blink bursts can be detected with good precision, and the state of drowsiness can be assessed with good precision.
