A three-dimensional CG manipulation apparatus has a real object, a position and orientation sensor provided to the real object, a support base, a position and orientation sensor provided to the support base, a head-mounted display, a position and orientation sensor provided to the head-mounted display, a sensor information acquisition unit for acquiring information from the sensors, a state determination unit for determining the current state of the real object and support base, a CG data management unit for managing CG data to be displayed, and an image generation unit for generating an image to be displayed on the head-mounted display. In a mixed realty system that presents an image formed by superimposing a virtual CG image on a real object to the user, a manipulation such as replacement or the like can be easily made for the virtual CG image without interrupting the user's operation.