An image acquisition unit (2) acquires an image obtained by capturing at least eyes of a target person. An object specifying unit (4) specifies an object in a line-of-sight direction of the target person using the acquired image. An incident light amount calculation unit (6) calculates an incident light amount representing an amount of light incident on the eyes of the target person using the acquired image. A reference pupil size determination unit (8) determines a reference pupil size based on the calculated incident light amount. A pupil size calculation unit (10) calculates a pupil size of the target person using the acquired image. An interest determination unit (12) determines an interest of the target person in the object by comparing the determined reference pupil size with the calculated pupil size.