PURPOSE: To prestress concrete by putting into the concrete prestress shrink fibers having the function of shrinking when heated to a certain temperature.

CONSTITUTION: Prestress shrink fibers 1 are mixed in fluid concrete. Engaging protrusions 1a, 1a are projected at both sides of each of the prestress shrink fibers 1 so that each of the fibers takes the form of a barbell. The fibers are thereby held in a coupled state with the concrete. When the prestress shrink fibers 1 are shrunk by heating to a certain temperature, the coupling portion 1b of each of the fibers 1 is pulled and the engaging protrusions 1a, 1a keep the concrete in engagement with the fibers. Considerable additional strength is thereby provided and costs can be reduced.