To facilitate the opening and re-assembling of a form at a time of demolding by providing an opening means passing a headed screw lever through front and rear cores to clamp front and rear side planks and pressing the front and rear side planks to left and right side panels from the inside.

When the revolvable head 151 of the clamp means 200 supported on a front side plank 50 by a thrust bearing 52 is rotated to the left at a time of the opening of a form, the rectangular clamp nut 160 clamping a front core 55 and rear core 65 is released in its pressing force to retreat and loosen to become a horizontal state by the revolution restricting piece 68 provided to the rear surface of a rectangular clamping hole 66 to stop the nut 160 horizontally and the clamp nut 160 becomes the state capable of passing through clamping hole 66. Next, when the revolvable heads 251 of the opening means 300 protruding from the front parts of both side plates 80, 80 are rotated to the left, the roller 270r of the leading end part of an L-shaped link 270 tumbles while presses front and rear side planks 50, 60 strongly from the inside and both cores 55, 65, a plurality of insertion hole forming members 70 and a concrete surface are instantaneously released to easily open the form.