To provide a simplified and noninvasive method for evaluating a subject's stress state, this stress evaluation method includes: (1) a step of instructing a subject to face an imaging device and smile to the maximum extent; (2) a step of capturing an image of the subject's smile with the imaging device; (3) a step of processing the captured face image and obtaining parameters that reflect stress; and (4) a step of calculating a stress index value by means of computation using the parameters obtained by processing the image. The method is characterized in that step (3) identifies the positions of the left and right outer corners of the eyes and the left and right corners of the mouth in the face image, draws a straight line (a1-b1) passing through an outer corner of the eye (a1) and a corner of the mouth (b1) on one side and a straight line (a2-b2) passing through an outer corner of the eye (a2) and a corner of the mouth (b2) on the other side, measures an angle ([Theta]1) between the straight line (a1-b1) and a reference line and an angle ([Theta]2) between the straight line (a2-b2) and the reference line, and that step (4) uses the values '[Theta]1' and '[Theta]2' as the parameters.
Stress evaluation method