An expanding device for expanding the spacing between bones is provided. The expanding device includes a spacer configured to be expandable and disposed between the bones, a catheter provided with the spacer at a distal end thereof and adapted to introduce a filler into the spacer, and a sheath having a lumen through which the catheter is inserted and in which the spacer before expansion can be housed. The spacer includes a spacer body configured to be expandable, an introduction section through which a filler is introduced into a rear-end-side expansion section of the spacer body, and a bypass passage through which part of the filler in the introduction section is introduced into a distal-end-side expansion section of the spacer body.