To obtain a fireproof coating layer being excellent in durability and exhibiting excellent adhesion even when the constituent foamable fireproof coating material is exposed to a fire by undercoating a steel structure with a rust-inhibiting coating material containing a rust-preventing pigment comprising a borate and/or a phosphate and forming a fireproof coating layer by using a foamable fireproof coating material.

The rust-preventing pigment used is a borate such as barium metaborate, strontium borate, or zinc borate or a phosphate such as aluminum tripolyphosphate, zinc phosphate, calcium phosphate, or calcium phosphomolybdate. The binder of the rust-preventing coating material is exemplified by a synthetic resin or an alkyl silicate. The synthetic resin used is exemplified by an acrylic resin, alkyd resin, vinyl chloride resin, urethane resin, epoxy resin, silicone resin or polyester resin. The alkyl silicate used is exemplified by ethyl silicate or butyl silicate.