An expanding device (10) for expanding the spacing between bones is provided. The expanding device (10) includes a spacer (18) configured to be expandable and disposed between the bones, a catheter (16) provided with the spacer (18) at a distal end thereof and adapted to introduce a filler into the spacer (18), and a sheath (12) having a lumen (14) through which the catheter (16) is inserted and in which the spacer (18) before expansion can be housed. The spacer (18) includes a spacer body (24) configured to be expandable, an introduction section (26) through which a filler is introduced into a rear-end-side expansion section (36) of the spacer body (24), and a bypass passage (28) through which part of the filler in the introduction section (26) is introduced into a distal-end-side expansion section (38) of the spacer body (24).
