There is provided a flaw detection testing method applied to a slot dovetail portion of a turbine generator rotor shaft. The flaw detection testing method includes an angle determining step of determining a slot angle of the slot dovetail portion using a variable-angle ultrasonic probe, a flaw detection performing step of, on the basis of a determination result in the angle determining step, using an angle ultrasonic probe or a phased array probe to perform flaw detection to detect a flaw in the slot dovetail portion, and a flaw depth measuring step of, when the flaw is detected in the flaw detection performing step, using an angle ultrasonic probe to measure a depth of the flaw from a surface of the slot dovetail portion.