A prewhirl type inlet casing for inducing a spiral stream in fluid comprising 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 is provided in the fluid passage in the inlet casing. The guide vane 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 has a rectifying function capable of causing the fluid to deflect into the swirling direction of the swirl stream induced by the internal passage. With these rectifying functions, a rectified swirl stream can be easily created in the in internal passage so as to prevent occurrence of a deviation of a cavitations inducing zone. With this configuration, it is possible to effectively prevent occurrence of the deviation of the cavitations inducing zone in order to aim at simplifying the configuration of the internal passage.