1. A display device, comprising: a display unit for stereoscopically displaying a display object to appear as an object existing in a three-dimensional space; a detecting unit for detecting an actual object in the three-dimensional space; and a control unit for changing, when a movement of the actual object is detected in the three-dimensional space, a display form of the same display object according to the movement of the actual object and a type of the display object, wherein the detecting unit is configured to detect a state in which the actual object moves continuously from outside of the display object to inside thereof for a longer period of time than a predetermined time, the control unit is configured to start changing the display object from when the movement of the actual object to the inside of the display object is detected, and the control unit is configured to display, when the movement of the actual object to the inside of the display object becomes undetectable before the predetermined time elapses and after the display object is started to change, a reverse change such that the change of the display object is returned to its original state.