A constant-force device includes a first ratchet wheel mounted rotatably on a shaft and having first sawtoothed projections, a second ratchet wheel mounted on the shaft so as not to rotate relative to the shaft but to move along the shaft and having second sawtoothed projections opposed to the first sawtoothed projections, an intermediate ratchet wheel mounted on the shaft so as not only to rotate relative to the shaft but also to move along the shaft between the first ratchet wheel and the second ratchet wheel and having a pair of intermediate sawtoothed projections meshing with the first and second sawtoothed projections respectively, a biasing member configured to bias the second ratchet wheel towards the first ratchet wheel, and a lock mechanism configured to set the first and intermediate ratchet wheels in a connected or unconnected state.