In a casing of a hydraulic machine of the type comprising a stay ring including a pair of opposing annular plates and a plurality of stay vanes fixed therebetween to form stay vane flow passages between the stay vanes, and a spiral case disposed on the radially outside of the stay ring, the stay vane passages and the spiral case are proportioned such that the ratio n of the area 2.pi. R.sub.o B of a circle encircling the outer periphery of the stay vanes and the cross-sectional area .pi. R.sub.s.sup.2 of the flow passage in the spiral case is from 1.9 to 2.5, and that the ratio (R-R.sub.o) /R.sub.s =l is larger than 0.75 where R.sub.s represents the radius of the cross-section of the flow passage in the spiral case taken along a plane containing the axis of the shaft of the hydraulic machine and intersecting at right angles with the axis of an inlet to the spiral case, R the distance between the axis of the shaft and the center of the flow passage in the spiral case, R.sub.o the radius of the stay vanes to the outer periphery thereof, and B the height of the stay vane flow passages at the periphery of the stay vanes.