Characteristics of an extremely fast pump-down process were studied in an aluminum ultrahigh vacuum system, using super-dry nitrogen. The system is constituted of a roughing pump line, a main pump line, a venting gas line and a main chamber with vacuum gauges. The venting gas line has two types of water traps in series. Experiments were done after achieving 10(-11) Torr range following a bake. Experimental results were the following : (1) using only the moisture trap in venting (case A), the pressure reached, from 760 Torr, was 1 X 10(-5) Torr in 0.7 min, 1 X 10(-8) Torr in 4.2 min, 1 x 10(-10) Torr in 1000 min; the pressure was proportional to exp(-St/V) down to the 10-5 Torr range; here, V, S, and t are the chamber volume, the pumping speed, and the pumping time, respectively; (2) using both the moisture trap and the "MonoTorr" in venting (case B), the pressure reached from 760 Torr was 1 X 10(-5) Torr in 0.7 min, 1 X 10(-8) Torr in only 2.5 min, 1 x 10(-10) Torr in only 170 min; the pressure was proportional to exp (-St/V) to the 10(-5) Torr range. The difference of results between (1) and (2) is mainly due to the difference of the amount of water and hydrogen content in vent-gas; the moisture trap can reduce the H2O content of vent-gas to 200 ppb after 2 h of operation, on the other hand, the MonoTorr can reduce other contents (H2O, H-2, CO, CO2, etc.) of vent-gas, except N2, to a few ppb after 2 h of operation.