The present invention achieves highly reliable optical-fiber loss adjustment at low cost, with easy work. An optical fiber is inserted from an optical fiber insertion opening into a space between an internal wall and a plurality of optical-fiber support members and, due to its self-resilience, comes in contact with the support members on its outer side. Since the radius of curvature of the fiber is sufficiently small, a loss can be adjusted with the number of turns of the fiber. The support members have a cylindrical shape and therefore come in point contact with the fiber, which can prevent microbending. Moreover, the interval (angle) between adjacent ones of the support members and the radius thereof are set such that the fiber can be led into and out of an optical-fiber adjustor, with no stress imposed.