A flexible polyurethane foam is obtained by making a material containing polyols, a cross-linking agent and a polyisocyanate compound react in the presence of a catalyst, a blowing agent and an auxiliary blowing agent. At this time, the isocyanate index of the polyisocyanate compound is set to 110 to 120. Preferably, a polyol of which the hydroxyl value is 250 mg KOH/g to 650 mg KOH/g and of which the molecular weight is 150 to 500 is used as the cross-linking agent, and concretely, polyethylene glycol can be used. A liquefied carbon dioxide is used as the auxiliary blowing agent with an amount mixed in of 1.5 mass parts to 6.0 mass parts relative to 100 mass parts of the polyols. The load required for compressing to 25% of the initial thickness is 30 N to 70 N, and thus, the flexible polyurethane foam has a low hardness.