To provide a fire preventive coating agent for a rigid cellular polyurethane insulating layer excellent in execution property and fire proof at low cost, using inexpensive water glass, and an in-place foaming method for the rigid cellular polyuretahne insulating layer, using the same.

This in-place foaming method for the rigid cellular polyurethane insulating layer is a method for forming the rigid cellular polyurethane insulating layer on a structure surface, by spray-foaming a polyisocyanate component and a polyol component onto the structure surface, and is carried out by depositing, on an insulating layer surface, the fire preventive coating agent containing an aqueous silicate solution, and 0.1-10 pts.wt. of surfactant and 1-70 pts.wt. of clay mineral thickener per 100 pts.wt. of solid content of the aqueous silicate solution.