Scroll casing and centrifugal compressor

A scroll casing of a centrifugal compressor includes a scroll part forming a scroll passage of the centrifugal compressor. The scroll part has a near-circular scroll cross-section which includes a first arc portion extending from a connection position with a hub-side passage surface of a diffuser passage of the centrifugal compressor (first position) to a one-direction side (side toward fourth position), a second arc portion formed on the one-direction side of the first arc portion so as to include at least a part of a region between an outermost end in the radial direction (second position) and an innermost end in the radial direction (fourth position), and a third arc portion formed on the one-direction side of the second arc portion so as to include an end position of the scroll part on the one-direction side (fifth position) and satisfies a relationship of R2>R3, where R2 is a curvature radius of the second arc portion, and R3 is a curvature radius of the third arc portion.