To efficiently detach a form panel which is fixed by means of a separator, after the placing of concrete.

A separator through-hole as a through-hole with a shape and a size, which enable the separator to move in a direction perpendicular to a central axis, is formed in a sheathing board. The separator is provided by means of the form panel with the sheathing board in such a manner that its leading end is protruded to the outside of the sheathing board from the separator through-hole. The form panel is held in the state of being sandwiched between an inner-surface sandwiching member which is engaged with a screw portion of the separator so as to abut on the inside surface of the sheathing board, and an outer-surface sandwiching member which is engaged with the screw portion of the separator so as to press an outside surface of the sheathing board. The inner-surface sandwiching member has a size and a shape, suitable for closing the separator through-hole.