To prevent a boundary layer from removing from the pressure surface side of a blade to a negative pressure surface side while crossing an inner side flow passage by providing with a riblet whose height is nearly equal to the thickness of the boundary layer of a side wall surface and which is made gradually larger as approaching to an impeller outlet, on the side wall surface of the inner side flow passage.

Riblets 3 whose length is low and having a thickness of a boundary layer, are arranged along a main flow direction on a hub wall surface 2 and a shroud wall surface 4. The height of the riblet 3 is formed smaller or in 0 size on an inlet side matching with the thickness of the boundary layer, and it is made larger as approaching to an outlet from the inlet of the impeller. In the riblet 3, it is possible to the boundary layer from removing from the pressure surface side of the impeller 1 to a negative pressure surface side while crossing an inner side flow passage, and wind makes flow in the same direction as main flow. It is thus possible to prevent the boundary layer from enlarging, and also it is possible to improve efficiency of the impeller.