PURPOSE: To facilitate the shaping, removal and movement of forms with simple structure.

CONSTITUTION: An inner form device for construction of hollow concrete structure is formed of a pair of form bodies A, B with connecting forms 2, 2 connected to both ends of center part forms 1 by hinges 6. Both form bodies A, B are shaped into hollow frame shape with the tips of the respective connecting forms 2, 2 colliding with each other. After the hardening of placed concrete, the respective connecting forms 2, 2 are rotated inward, and then the center part forms 1, 1 are pulled inward to remove and contract the whole form bodies A, B. In this state, the form bodies A, B are respectively moved into the following construction position.