A suction casing comprising: a casing main unit; a suction nozzle that is provided in a manner protruding to the outer circumferential side of the casing main unit in a radial direction and introduces a fluid from an outer circumferential side to an inner circumferential side in the radial direction; a chamber that comprises a substantially doughnut-shaped space in communication with an inside of the suction nozzle on the outer circumferential side, that guides the fluid, introduced from the suction nozzle, to an inlet opening portion opening in an axial direction and disposed in a substantially annular shape, and that includes an upper portion which functions as a joint portion to which the suction nozzle is joined and a lower portion which is located at a position opposite to the upper portion across the center of the rotary shaft; and a fitting part that is located in the chamber and is fitted into the chamber at a range of the lower portion from side portions of the chamber, wherein the chamber includes an introduction portion and a guide portion, the introduction portion being substantially annular and being disposed on the outer circumferential side in the radial direction, and the guide portion being substantially annular and communicating the introduction portion with the inlet opening portion of an impeller, characterized in that a radial width of the introduction portion has a width that is constant from a top portion of the chamber to side portions of the chamber, which are 90 degrees away from the top portion of the chamber in a circumferential direction, and the radial width gradually narrows in the circumferential direction along an outer peripheral surface of the fitting part in the range from the side portions of the chamber to the lower portion. a casing main unit; a suction nozzle that is provided in a manner protruding to the outer circumferential side of the casing main unit in a radial direction and introduces a fluid from an outer circumferential side to an inner circumferential side in the radial direction; a chamber that comprises a substantially doughnut-shaped space in communication with an inside of the suction nozzle on the outer circumferential side, that guides the fluid, introduced from the suction nozzle, to an inlet opening portion opening in an axial direction and disposed in a substantially annular shape, and that includes an upper portion which functions as a joint portion to which the suction nozzle is joined and a lower portion which is located at a position opposite to the upper portion across the center of the rotary shaft; and a fitting part that is located in the chamber and is fitted into the chamber at a range of the lower portion from side portions of the chamber, wherein the chamber includes an introduction portion and a guide portion, the introduction portion being substantially annular and being disposed on the outer circumferential side in the radial direction, and the guide portion being substantially annular and communicating the introduction portion with the inlet opening portion of an impeller, characterized in that a radial width of the introduction portion has a width that is constant from a top portion of the chamber to side portions of the chamber, which are 90 degrees away from the top portion of the chamber in a circumferential direction, and the radial width gradually narrows in the circumferential direction along an outer peripheral surface of the fitting part in the range from the side portions of the chamber to the lower portion.