A reflecting surface design system (50) has (1) first rendering means (54) for displaying a free-form surface (20) on which a plurality of segments (24), each of which is defined by a plurality of vertices (25.sub.1 to 25.sub.4), are formed, (2) reflection information specifying means (56) for specifying the vertex position and light reflecting direction for each of the plurality of vertices (25.sub.1 to 25.sub.4) that define one of the plurality of segments (24), and (3) surface calculation means (58) for calculating a surface (S) to be assigned to the one segment (24) on the basis of the vertex positions and light reflecting directions specified for the plurality of vertices (25.sub.1 to 25.sub.4) that define one segment (24), and the light source position, which is specified in advance.