According to one implementation, a fiber width adjustment device includes: a feeder and an adjuster. The feeder feeds a tape material in a length direction of the tape material. The tape material consists of fibers for a fiber reinforced resin after or before the fibers are impregnated with a resin. The adjuster has a path for the tape material. The path is formed by at least a bottom and a pair of wall surfaces. The interval of the wall surfaces decreased gradually. The width of the tape material which passed the path is changed by adjusting a part of the path. The tape material passes through the part of the path while contacting with the bottom and the wall surfaces.