The following steps are included: an alignment step in which a phased-array probe that contains a plurality of transducers that are each capable of emitting ultrasound is aligned against an end surface of the rotor disc such that the plurality of transducers are in a parallel state, lined up in the circumferential direction of the rotor disc; a first transmission/reception step in which ultrasound is emitted from the plurality of transducers in the aforementioned parallel state with the timing of the ultrasound emission from the respective transducers controlled in accordance with a first emission pattern and the resulting reflected ultrasound is received; and a second transmission/reception step in which ultrasound is emitted from the plurality of transducers in the abovementioned parallel state with the timing of the ultrasound emission from the respective transducers controlled in accordance with a second emission pattern that is different from the aforementioned first emission pattern and the resulting reflected ultrasound is received.