A process for producing a rigid foamed synthetic resin, which comprises reacting a polyol mixture (P) containing a polyol (A) and a polyol (B) and a polyisocyanate compound (I) in the presence of a blowing agent containing water, a catalyst, a foam stabilizer and a flame retardant. The polyol (A) is a polyether polyol having a hydroxyl value of from 56 to 250 mgKOH/g, obtainable by subjecting an alkylene oxide to ring-opening addition polymerization to a bisphenol compound. The polyol (B) is a polyether polyol having a hydroxyl value of from 100 to 800 mgKOH/g, obtainable by subjecting an alkylene oxide to ring-opening addition polymerization to a Mannich condensation product.