1. An ultrasonic defect testing method for testing an object having a three-dimensional curved surface comprising the steps of: (a) measuring a three-dimensional shape of the object under test to acquire shape data representing the shape thereof; (b) measuring a defect of the object under test by three-dimensionally scanning the object with an ultrasonic probe to obtain defect test data representing a defect testing result; (c) monitoring the ultrasonic probe to contactlessly measure a three-dimensional position of the ultrasonic probe to obtain positional information of the ultrasonic probe; and (d) displaying on a display screen a combined image in which, by utilizing the positional information of the ultrasonic probe, a three-dimensional graphic image of a defect zone prepared using the defect test data is superimposed on a three-dimensional graphic image of the object under test prepared using the shape data.