TITLE OF THE INVENTION: GLOSSY FIBER TECHNICAL FIELD [0001] The glossy fiber of the present invention has such excellent properties that the glossy fiber not only has a deep lustrous gloss imparted by regulating the average reflectance, average transmittance, and contrast gloss in a visible-light region but also can be processed into a woven or knit fabric suitable for garment applications. BACKGROUND ART [0002] Synthetic fibers made of polyesters, polyamides, or the like have excellent mechanical properties and dimensional stability and are hence in extensive use in applications ranging from garment applications to non-garment applications. Nowadays, however, people have come to live diversified lives and to desire better lives, and there is hence a desire for fibers having a high degree of sense or functions not possessed by any conventional synthetic fibers, in many applications including garments. [0003] With respect to the development of techniques regarding synthetic fibers, it is not too much to say that the progress of elementary techniques therefor has been made by imitating natural materials as a motivation. For example, in order to obtain the gloss peculiar to a natural material, investigations have been extensively made on techniques ranging from polymer techniques to fiber formation techniques including design of the cross-sectional shapes of fibers. [0004] This is because the gloss of a natural material is more complicated and fascinating and has a high-grade sense as compared with the monotonous glosses possessed by single fibers. The followings have been disclosed as fiber techniques aiming at obtaining a lustrous gloss produced by the complicated structure of a natural material. [0005] For example, Patent Document 1 discloses a synthetic fiber which has a noncircular cross-section and improved light-reflecting surface properties due to the cross-section and which has a thus imparted gloss such as that of silk, which is natural fibers of high rank. [0006] Patent Document 2 discloses a fiber that has a noncircular cross-section and contains fine voids inside so that the noncircular cross-section and light reflection due to the fine voids produce a synergistic effect, which enables the fiber to have a high-grade gloss similar to that of natural silk. [0007] Patent Document 3, for example, discloses a golden or silvery filament obtained by vapor-depositing a metal on a fiber itself and a metal-coated slit filament obtained by vapor-depositing a metal on paper or a film and slitting the metal-coated paper or film, for the purpose of imparting a deep lustrous gloss to synthetic fibers or to woven or knit fabrics configured of synthetic fibers. [0008] In Patent Document 4 and Patent Document 5, the phenomenon in which a fine structure represented by ones in buprestids or morpho butterflies produces a color is utilized to propose a structurally colored fiber which has an accurately controlled cross-sectional shape to thereby have any desired color in the visible-light region. [0009] In Patent Documents 4 and 5, two polymers differing in refractive index are alternately superposed to form an alternating multilayer structure while accurately controlling the number of superposed layers and the thickness of each layer, thereby making it possible to impart structural coloring due to the interference and reflectance of light. Unlike the conventional coloring by dyes, the structural coloring thus obtained is expected to produce both a high gloss and a deep color tone. BACKGROUND ART DOCUMENT PATENT DOCUMENT [0010] Patent Document 1: JP-B-36-20770 (claims) Patent Document 2: JP-A-2006-161218 (claims) Patent Document 3: JP-A-2002-307602 (claims) Patent Document 4: JP-A-7-34324 (claims) Patent Document 5: WO 1998/46815 (claims) SUMMARY OF THE INVENTION PROBLEMS THAT THE INVENTION IS TO SOLVE [0011] Although there are techniques concerning fibers having a gloss similar to natural glosses, such as those disclosed in Patent Documents 1 and 2, no fiber which has a deep lustrous gloss, such as those of natural metals having a high-grade sense, and which sufficiently satisfies properties required for use in garment applications has been obtained hitherto. [0012] The method in which a metal is vapor-deposited on a fiber, disclosed in Patent Document 3, etc., has a drawback in that the thin metal film may crack due to friction caused by fiber processing, such as twisting or knitting/weaving, or by laundering, resulting in loss of the gloss. In addition, to vapor-deposit a metal on individual fibers is exceedingly poor in production efficiency. Evan in the case of the relatively efficient production method in which a metal is vapor-deposited at a time on a film or the like and the coated film or the like is slit, the slit filaments are undesirably thick and flat as compared with the fibers in ordinary use in garment applications and, hence, the woven or knit fabric obtained therefrom often has a problem in that the fabric is poor in softness. [0013] Furthermore, the fibers proposed in Patent Documents 4 and 5 have a cross-section including superposed platy structures and, hence, naturally has a flat contour because of the superposed platy structures included therein. Moreover, since the superposed platy structures are superposed layers of incompatible polymers, interlaminar separation is prone to occur and it is necessary to dispose a thick protective layer around the multilayer structure in order to prevent the interlaminar separation. Because of this, there are considerable limitations on fiber processing, the possible structure of the fibric, etc. and, above all, there is a problem in that the single filaments have a larger diameter and, hence, the fabric produced from the composite fiber has an exceedingly stiff feeling and is poor in softness. [0014] The fibers of Patent Documents 4 and 5 further have a drawback in that the arrangement of single filaments is disordered by fiber processing, etc. and, hence, the structurally colored fiber is possible only with a limited structure capable of yielding the intended bundle of fibers having a uniform cross-section. Because of this, it is difficult to sufficiently obtain the expected structural coloring when the composite fiber is used to merely produce a simple woven or knit fabric. There also is, for example, surface reflection due to the thick protective layer. Consequently, there are often cases where a visible coloration is not obtained, and it has been difficult to apply the composite fiber to garment textiles appealing the aesthetic properties. [0015] An object of the present invention is to provide, in view of those problems of background-art techniques, a glossy fiber which has a deep lustrous gloss and can be processed into a woven or knit fabric suitable for garment applications. MEANS FOR SOLVING THE PROBLEMS [0016] The object of the present invention is achieved by the following configurations, namely: (1) A glossy fiber having, in a visible-light wavelength region, an average reflectance of 20% or higher, an average transmittance of 40% or less, and a contrast gloss of 3.0 or less; (2) The glossy fiber according to (1), having a cross-section along a direction perpendicular to a fiber axis, the cross-section having an inscribed circle diameter RB and a circumscribed circle diameter RC for the fiber which have a relationship represented by 1.0