An apparatus of the supersonic molecular beam with a small pumping system has been constructed. Beam intensity and pressures in the nozzle, buffer and collision chambers were measured as a function of nozzle-skimmer distance for various nozzle pressures. The maximum beam intensity was obtained at nozzle-skimmer distances around 3 mm, and was 40 times as high as that of the effusion beam at nozzle pressure 720 Torr. The effects of background pressures in the nozzle and buffer chambers have been investigated and it is shown that the conductance in the non-equilibrium state must be considered in the present system.