A reinforcement device for an optical fiber fusion-spliced portion, which reinforces a fusion-spliced portion of optical fibers by heating and shrinking a reinforcement sleeve covering the fusion spliced portion, includes a heater configured to heat the reinforcement sleeve. The heater includes a sleeve housing portion capable of housing the reinforcement sleeve. The sleeve housing portion includes a first wall portion extending in a longitudinal direction of the sleeve housing portion and a second wall portion facing the first wall portion. The first wall portion and the second wall portion are configured such that a distance therebetween increases from a bottom portion side of the sleeve housing portion toward a top portion side of the sleeve housing portion in a cross-section orthogonal to the longitudinal direction. At least one bent portion is formed to at least one of the first wall portion and the second wall portion in the cross-section.