To fit the characteristic of a pump to a desired specification by a simple replacing operation.

The water of which the pressure is increased by an impeller 3, is discharged to a downstream through a guide impeller vane 6. A part at an upstream side with respect to a peak part of the guide impeller vane, of the guide impeller vane, is a partial guide impeller vane 6a, and this partial guide impeller vane 6a can be replaced. The partial guide impeller vane 6a to be fitted to the desired specification, is selected from plural kinds of partial guide impeller vanes having the vane inlet angles and the effective passage widths different from each other, to be mounted. That is, in a case when the performance is improved at a large capacity side, the partial guide impeller vane 6a of large vane inlet angle and large port is used, and in a case when the performance is improved at a small capacity side, the partial guide impeller vane 6a of small vane inlet angle and small port is used.