Abstract: A method of measuring the inclining angle of a planar defect of a solid material with ultrasonic waves which includes the steps of irradiating ultrasonic waves incident to the planar defect of a solid material while longitudinally scanning a probe forwardly and backwardly. The inclining angle of the planar defect is determined based on the inclination of an echo envelope obtained from the relationship of the echo beam path distances of the ultrasonic waves versus the echo amplitudes or heights of the reflected waves reflected from the planar defect of the solid material. The inclining angle of the defect corresponds to a straight line portion of the echo envelope in a region where the echo height decreased gradually from a maximum height position as the echo beam path distance increases.