PURPOSE: To bond each element, reducing loss of ultrasonic energy and improving efficiency, by holding a thermoplastic resin made element of which a plurality of blades is arranged cylinder like to a supporting member in a vertical direction by use of a specified guide cylinder and applying an ultrasonic wave.

CONSTITUTION: A pluraity of blades 11 is fixed annulus like and vertically to one face of a supporting member 13, meanwhile an element 10 is shaped by preparing an engaging groove 12 of the blade 11 on another face. Then, a desired number of said elements 10 is heaped up in its axial direction, a tip of the blade 11 being made to fit in the engaging groove 12 of a neighboring element, meanwhile a guide cylinder 5 is fitted to an outside of each element 10, the necessary minimum of a clearance between the supporting member 13 and a guide cylinder being kept, the element 10 is held as vertically as possible. Next, an assembly being pressed in the axial direction and being applied with ultrasonic energy from a horn 40, each element 10 is heat-bonded each other and a cylindrical impeller is shaped.