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A diagnostic system using optical fibers with high numerical apertures is constructed for investigating a structure of magnetically confined plasma. Since the fiber has no convex lens at its front facet, the radiation from all plasmas included in the viewing angle of the fiber is utilized. First, theoretical forms are derived to determine the structure of the plasma from optical signals of a few fibers arranged axisymmetrically around the plasma. Second, the numerical aperture is measured by a simple method using a circular fluorescent lamp. Third, the theoretical forms are ascertained by a mock-up experiment where a light source representing a model plasma is constructed by a matrix array of straight fluorescent lamps. In this experiment, the effect of a reflected light from a wall of a vacuum vessel is investigated. Finally, the optical system is applied to a field-reversed-configuration plasma. Obtained structures of the plasma for n=1 and 2 toroidal modes are compared with the results of the conventional method using the fibers with convex lenses. The former results agree well with the latter ones in spite of the use of one-sixth the number of fibers in the latter method. [ABSTRACT FROM AUTHOR]