To prevent leakage from the joining part of an outer frame by making the outer frame openable/closable by an elastic connection plate and sealing the joining part of the outer frame watertightly by the connection plate.

A form device has an inner frame 1, an outer frame installed openably/closably on both side in the width direction of the inner frame 1, and end plates which are installed on both end sides in the longitudinal direction of the inner frame 1 and have an expandable/contractable distance. An outer frame 3 is connected with both end parts in the width direction of the inner frame 1 by an elastic metal connection plate 33. A seat member 2 has a stopping member 5 which contacts and stops the outer frame 3 when it is rotated outside to be opened at a required angle. Closing members which guide the outer frame 3 to rotate inside and close it by decreasing the distance between the end plates are installed in both end parts in the width direction of the end plates.