casing 2 b includes an inclined wall section not shown and the trailing end of the inclined wall section on the shroud casing 2 a side is formed to be closer to the outside in the radial direction than the trailing end of the inclined wall section of the hub casing 2 b gently protrudes to sh side in addition the shroud side protrusion length sd of the wall section 31 of the shroud casing 2 a is 40 of the width w of the discharge outlet 6 b in the impeller 6 and a ratio d w between the flow path width d of the vaneless diffuser 12 and the width w of discharge outlet 6 b is 0 6 in addition in comparative 1 the fluid circulation flow path 21 is not provided in comparative example 2 the shroud side protrusion length sd of the wall section 31 of the shroud casing 2 a is changed and a corner r portion having a small curvature radius r is only provided between the wall section 31 of the shroud casing 2 a and the horizontal wall steeply protrudes to sh side the shroud side protrusion length sd is 17 of the width w of the discharge outlet 6 b.