1. A head-mounted display device comprising: a display; a single camera configured to acquire images that include at least part of an outside scene in a visual field direction of the user; and a processor or an application specific integrated circuit (ASIC) configured to: repeatedly acquire, from a motion sensor, motion information regarding a head of a user wearing the head-mounted display device; and when the motion of the head is rotational movement about the axis of the head of the user: (A) calculate, based on acceleration information included in the acquired motion information, a rotation amount of the head starting from a point in time when the first image is acquired by the single camera; (B) calculate, based on the calculated rotation amount of the head of the user, a rotational movement amount of the single camera starting from the point in time when the first image is acquired by the single camera; (C) determine whether the calculated rotational movement amount of the single camera starting from the point in time when the first image is acquired by the single camera is equal to or larger than a predetermined movement amount threshold; and (D) when the calculated rotational movement amount of the single camera starting from the point in time when the first image is acquired is equal to or larger than the predetermined movement amount threshold: (i) instruct the single camera to acquire a second image; (ii) estimate, on the basis of the first and second images, a position of a real world target object present in the outside scene; and (iii) cause the display to display a virtual image including a virtual object added to the real world target object at the estimated position of the real world target object.