A fiber sensor comprising: an optical waveguide unit having a measurement surface formed at an end of an optical waveguide; and a channel unit through which a specimen, serving as a measurement object to be measured by contact with the measurement surface, is caused to flow in and out via a specimen channel, wherein the optical waveguide is fixed to the optical waveguide unit such that the measurement surface forms a part of a channel wall surface of the channel unit, and the optical waveguide unit is provided so as to be detachable from the channel unit.