In contrast with relatively simple cross-sectional shapes, it is difficult to discern misalignments in relative position and angular orientation between mating parts with complicated cross-sectional shapes. This is because geometrical uncertainties depend on the complexity of the shapes, and their assembly requires complete information about possible misalignments along their mating boundary interfaces, This motivates the development of a new sensing method for detecting misalignments, However, in the area of assembly, such a method has not been fully developed; for the most part, only local sensing techniques using proximity, tactile, force, and vision sensors have been developed. In this paper, a novel omnidirectional image sensing system for assembling parts with arbitrary cross-sectional shapes is proposed, and its features are investigated theoretically. Its feasibility for assembly is also shown by experiments, Utilizing a pair of conic mirrors and a camera, this system can immediately acquire a 2 pi coaxial image of the misalignment along the mating boundary interface between mating parts without experiencing self-occlusion.