PURPOSE: To surely avoid weighing troubles in the weighing of a polyol-gas dispersion and to control the number of cells by dispersing gas in a component between a measuring pump and a mixing head. CONSTITUTION: To a mixing head 1, an isocyanate is supplied via a measuring pump 2, and a polyol is supplied via a measuring pump 3 corresponding to a necessary isocyanate index. Moreover, through an arrow 4, other additives are supplied. A dispersion device 5 is installed between the mixing head 1 and the pump 3. Since the partial flow of the polyol for dispersion is used, a branch connection 31 is formed behind the pump 3, and further a partial flow pump 32 is installed in a part reaching the dispersion device 5. The control of the number of cells of polyurethane foam is made by supplying the polyol to the dispersion device by the pump 32 and a necessary amount of air via a control valve 6.