PURPOSE: To set the respective tension members to the optimum positions along a formwork by separating the input positions and the directions of both force for clamping the formwork and force for maintaining a clamping state and independently providing the tension members, the input part of clamping force and the input part of engaging force correspondent thereto.
CONSTITUTION: Force directed upward from the downside is inputted to the input part 14 of clamping force. Force directed downward from the upside is inputted to the input part 15 of clamping force. When engaging force is inputted to the input part 18, an advancing and retreating member 11 is advanced. A lock piece 13 is advanced integrally therewith and inserted into the rear surface of a head part 9. After the engagement of the lock piece 13, both clamping force inputted to the input parts 14, 15 and engaging force inputted to the input part 18 are released. Thereby the clamped state of a formwork is maintained by the lock piece 13. The formwork is supplied to a forming stage of concrete or the like in this state. Release of clamping is performed by inputting clamping force to the input parts 14, 15 and loosening the gap between the rear surface of the head part 9 and the upper-face door of a second projecting piece 5 and thereafter inputting releasing force to an input part 19.