PURPOSE: To obtain the gap simply as designed by a method wherein a cutting jig is inserted into a pump casing so that the flange for the jig is abutted against the pump casing, in order to cut the blade of an impeller.

CONSTITUTION: When the cutting jig 10 is inserted into the pump casing 4 as the impeller 2 is being rotated, the end face 12 of the jig abuts against the blades 13, 13 of the impeller and the projecting stripes 14 of each blades are cut. The jig 10 is inserted until the flange 11 of the cutting jig 10 is abutting against the pump casing in order to cut the projecting stripes 14, thereafter, the jig 10 is pulled out, a casing cover 5 is fitted to an opening 8 and is attached thereto by screws. As a result, the gap between the casing cover 5 and the impeller 2 shows a value determined by the design thereof.