1. A three-dimensional image processing apparatus for generating a three-dimensional image perceived by one or more users based on two-dimensional viewpoint images corresponding respectively to right and left eyes of the one or more users, comprising: a storage unit having pre-stored front and back boundary positions of a three-dimensional object perceived by the user, the front boundary position being a limit up to where the three-dimensional object appears to be maximally close to the user and the back boundary position being a limit up to where the three-dimensional object appears to be maximally distant from the user; an instruction acquiring unit configured to receive a user instruction selecting front and back boundary positions from the pre-stored front and back boundary positions; and a parallax specifying unit configured to specify the parallax based on the front and back boundary positions specified in the user instruction.