A positional data acquisition unit of an action detector acquires positional data indicating the position of an image of a light-emitting part of a light-emitting device held by a user in an image frame at each time step, and also acquires curve data for the head contour at each time step estimated as a result of visual tracking by a tracking processor. A history storage unit stores a history of the positional data for an image of a light-emitting part and the curved data for the head contour. A determination criteria storage unit stores the criteria for determining that a predefined action is performed by referring to the time-dependent change in the relative position of the image of the light-emitting part in relation to the curve representing the head contour. An action determination unit determines whether the action is performed based on the actual data.