A process for producing a finely cellular polyurethane foam by mixing a first ingredient comprising an isocyanate compound and a second ingredient comprising a compound containing an active hydrogen group, characterized by comprising adding a nonionic silicone surfactant containing no hydroxyl group to at least one of the first ingredient and the second ingredient in an amount of 0.1 to 5 wt %, excluding 5 wt %, based on the total amount of the first ingredient and the second ingredient, subsequently agitating the surfactant containing ingredient together with an unreactive gas, which has no reactivity to isocyanate group or active hydrogen group, to disperse the unreactive gas as fine bubbles to prepare a bubble dispersion and then mixing the bubble dispersion with the remaining ingredient to cure the resultant mixture and forming finely cellular structure into the resultant polyurethane foam by the fine bubbles of the bubble dispersion.