A video acquirer acquires video obtained by imaging, by an imaging element that moves in association with motion of a head of a user, an area including reflected light of each of light beams irradiated to either one eyeball of the user from two light sources of a first light source that moves in association with the motion of the head of the user and a second light source whose relative position is invariable with respect to a video presenter as an observation target for the user. A head movement estimator estimates the motion of the head of the user based on a relative position of the reflected light of the second light source with respect to the reflected light of the first light source in the video acquired by the video acquirer.