To facilitate assembly, at a field site, of an interior/exterior underlayer in a heat insulating panel also serving as a form in which an interior/exterior underlayer material is formed in an integrated manner.

The heat insulating panel serving as the form and also as the interior/exterior underlayer comprises a heat insulating plate and a sheathing board used in common, and an interior/exterior underlayer material which is detachably provided on the surface opposite to the sheathing board face of the heat insulating plate from the surface side in a manner of extending in the vertical direction of the heat insulating plate. The interior/exterior underlayer material preferably has linear projections projecting toward the heat insulating plate and extending in the vertical direction, and longitudinal channels are formed in the face of the surface side of the heat insulating plate to engage and fix the linear projections.