A prewhirl type inlet casing for inducing a spiral stream in fluid has a
suction passage arranged on an upstream side, being orthogonal to a
rotary shaft of fluid machinery, and an internal passage connected to the
suction passage, the internal passage being formed in a spiral shape so
as to induce a swirl stream orthogonal to the rotary shaft in the fluid.
A guide vane provided in the fluid passage in the inlet casing has a
rectifying function capable of distributing flow rates in the swirl
stream of the fluid in the internal passage between the swirl center side
and the swirl outer peripheral side, and causing the fluid to deflect
into the swirling direction of the swirl stream induced by the internal
passage.