An in-situ foaming system for forming a flame-retardant polyurethane foam in situ comprising a first liquid containing a polyisocyanate (A), a second liquid containing a polyol (B), a trimerization catalyst (C), a foaming agent (D), a foam stabilizer (E), and additives (F) comprising red phosphorus and at least one member selected from the group consisting of phosphoric acid esters, phosphate-containing flame retardants, bromine-containing flame retardants, borate-containing flame retardants, antimony-containing flame retardants, and metal hydroxides.