Recently environmental problems have been taken up more and more widely in the world. In the automobile industry as well, severe control of fuel consumption and exhaust gas have spread globally after first imposed in the United States. Meanwhile, users' demand for driveability is still strong and compatibility of high power with low fuel consumption and clean exhaust gas is required at a high level.As one of the suitable solutions, the mechanically supercharged engine has got a highlight in the market because unlike turbocharged one it has the advantages of emission at start and response(1).Under these circumstances, automobiles equipped with a Miller cycle gasoline engine were put on sale by Mazda Motor Corporation for the first time in the world in 1993 as mass-produced automobiles(2). Ishikawajima-Harima Heavy Industries Company, Ltd. (IHI) jointly developed Screw type supercharger (Lysholm Compressor) for that engine as Fig.1.(3)(4) The Miller cycle engine is very effective in attaining both high power and low fuel consumption at the same time. To meet the requirement of high efficiency and high supercharging (boosting) from low speed, the screw type supercharger having internal compression,i.e., built-in pressure has been chosen as the best one of many types of superchargers.This supercharger as shown in Fig.2 and 3 has an advantage especially in the efficiency at the higher boost pressure compared with the conventional roots type supercharger. This is also effective as a hybrid system for diesel engines for example for truck use because high boost pressure can be obtained from low speed range(5). However, it is true that in the unload condition the screw type supercharger consumes a little larger driving power as a loss than roots type supercharger because the former has internal compression. Therefore, it is necessary to devise reduction of power loss at partial load, i.e., unloading range for conventional Otto cycle engines with relatively lower boost pressure. This paper introduces several methods to reduce driving work so-called parasitic loss of a screw type supercharger at an unloading condition.