To hold the fair face of concrete formed by a formwork in a flat state by constituting a formwork deformation preventing tool by the use of spacer member for preventing the flat plates of a formwork from being deformed in the direction of supporting members.

The height of a spacer member 20 is set nearly equal to the interval between supporting members 8, and on the places corresponding to small ribs 5 provided on the inner face side of the flat plate 1 of the spacer member 20, slits 21 of height so as to fit the small ribs 5 are provided on two positions front and rear, For fitting formworks T1, T2 by form ties 7, the spacer members 20 are mounted so as to be previously positioned on the circumference of the form tie fitting hole. The spacer member 20 is pinched with hands so as to deform the flat plate parts 20c on both ends into being in parallel with each other, the two ribs are fitted so as to engage with the slits 21 under the condition in which the U-shaped part is positioned on the circumference of the form tie fitting hole, force applied to the flat plate parts 20c is released, and hence the spacer member 20 is fitted in a locked state.