A manufacturing method of an ultrasonic generating device includes calculating performance value based on a first electromechanical coupling factor in thickness directions and a second electromechanical coupling factor in diametrical directions for each of existing piezoelectric elements, calculating, for each of temporary conditions, a temporary influence value on the basis of a deviation of temporary amplitude of the ultrasonic vibrations, generated by the supply of the current having the predetermined current value, from a target amplitude in a target condition. The manufacturing method includes selecting the corresponding mounted piezoelectric element to be mounted on each of the element mounting portions from the existing piezoelectric elements so that the sum of actual influence values of all the element mounting portions is within a predetermined range with respect to the target amplitude.