Method for producing polyurethane foam

The present invention relates to a method for efficiently producing polyurethane foam having a fine uniform cell structure, high mechanical strength and little difference in the mechanical strength among the sites. Such method does not cause the curing of material in a stirring mixer (a mixing head). The method for producing the polyurethane foam by a mechanical foaming method involves a process for stirring and mixing a polyol mixture containing a polyol and a catalyst with an isocyanate component in the premixer (A) without inert gas to prepare a urethane-forming liquid composition, feeding the obtained liquid composition and the inert gas to a stirring mixer (B), mechanically mixing the liquid composition in the atmosphere of inert gas to prepare a froth-shaped material wherein the inert gas is finely dispersed in the liquid composition, and extruding the froth material from the stirring mixer (B).