Impeller manufacturing method and impeller flow path elongation jig

An impeller manufacturing method includes: integrally forming an impeller by an additive manufacturing method using a metal powder, the impeller including a disk which has a disk shape about an axis, a plurality of blades which are formed on a surface facing a first side in an axial direction of the disk with gaps therebetween in a circumferential direction about the axis, and a cover which covers the plurality of blades from the first side in the axial direction; processing the integrally formed impeller by a hot isostatic pressing; and causing a polishing fluid containing abrasive grains to flow through a flow path formed between the disk, the cover, and the blades in the impeller after the processing with the hot isostatic pressing and while pressurizing the polishing fluid to perform fluid polishing.