To make it possible to change a moving stroke of a side frame in accordance with the stroke which varies even when such variations occur in order to release the side frame from a concrete molding.

A threaded rod 6 is hooked across a first and a second side frame 3, 4 assembled, in a freely slidable manner, into a bottom form 2, and a reverse thread part 6B and a forward thread part 6A to be screwed into the first and the second side frame 3, 4 for fixing, are formed on both ends of the threaded rod 6. In addition, the pitch of the right-hand thread part 6A to be screwed into the side frame 4 is set twince the pitch of the left-hand thread part 6B to be screwed into the other side frame 3. Thus the moving stroke of the second side frame 4 to be given by the rotation of the threaded rod 6 is twice the moving stroke of the first side frame 3.