A three-dimensional model modification system for modifying a three-dimensional model, defined by a plurality of surfaces and representing an arbitrary object is disclosed. The system includes a base-curve generating unit fitting skeleton lines, representing a prerecorded approximate outline of the object, to a three-dimensional model to be modified and generating base curves, representing an approximate outline of the three-dimensional model; a first polygon forming unit generating a group of points from the three-dimensional model; a base-curve modifying unit arbitrarily modifying the base curves of the three-dimensional model, according to an instruction from an operator; a group-of-points displacing/determining unit displacing the group of points based on base curve displacements from the base curves before arbitral modification to the base curves after arbitrary modification; and a second polygon forming unit generating a three-dimensional model defined by a plurality of surfaces formed from the group of points.