A helical-blade fluid machine maintains a low-pressure atmosphere in a closed casing (1) and secures a proper lubrication state for a compression mechanism (9). The fluid machine has a drive mechanism (7) that is installed at an upper or lower part of the casing and consists of a stator (11) and a rotor (15). The compression mechanism is installed at the other part of the casing. The compression mechanism has a cylinder (23) and a roller (25), which is swayed with respect to the cylinder by the drive mechanism. A helical blade (39) having unequal pitches is arranged on the roller, to define compression chambers (41). This arrangement lowers the head of lubricant. An intake pipe (5) feeds gas into the casing so that a low-pressure atmosphere fills the casing.