To obtain a inexpensive modified polyurethane foam having an improved flame retardancy without causing the formation of sea-island structures by previously mixing a polyisocyanate component with an active hydrogen compound under agitation and mixing the resulting mixture with an aqueous alkali silicate solution under agitation.

In producing a polyurethane foam composition by mixing a polyisocyanate with an active hydrogen compound component comprising active hydrogen compound having at least two active hydrogen groups and/or a mixture thereof and an active hydrogen compound having one active hydrogen atom and mixture thereof with adjuvants, such as a catalyst, additives, a foam stabilizer, a dispersant, a blowing agent and a flame retardant, used to modify the reaction, the properties of a product, etc., the following means is adopted. Namely, one component being one comprising an aqueous alkali silicate solution and/or the solution diluted with water is previously mixed with a polyisocyanate component and the active hydrogen compound (both of which include additives) under agitation, and the resulting mixture is mixed with an alkali silicate under agitation.