Action of a head around which an HMD main body is worn is related to manipulation of an object in an immersive three-dimensional virtual space. Information on a sight in a virtual space may be determined based on information on head inclination sensed with an inclination sensor; an image of the sight in the virtual space may be generated based on the sight information in order to display the sight image in the head mounted display; an object placed on a reference sight line in the virtual space may be identified, the reference sight line being determined in correspondence with a predetermined position in the sight image; and an object the identified object may be manipulated in response to the identification of the object in accordance with action of inclining in a predetermined direction the head around which the head mounted display is worn.