A process for producing a rigid polyurethane foam by use of a decreased amount of a CFC blowing agent and an increased amount of water used as a blowing agent, maintaining a high degree of heat insulation and improving the deterioration in friability of the product. The catalyst is one of the imidazoles represented by the formula (I): ##STR1## where R.sup.1 is alkyl having 1 to 4 carbons, benzyl, dimethylaminopropyl, or hydroxyalkyl having 1 to 3 carbons; R.sup.2 is hydrogen, alkyl having 1 to 4 carbons, allyl, or benzyl; and R.sup.3 and R.sup.4 are independently hydrogen, methyl, or hydroxymethyl.