The present invention provides a method and apparatus for an ultrasonic inspection of a penetration of a reactor pressure vessel, especially, a welded portion of the penetration. In the ultrasonic inspection, a position (inspection position) of an ultrasonic probe relative to a portion to be inspected can be identified without an effect of the surface of a portion to be inspected.The ultrasonic probe of the ultrasonic inspection apparatus, which is pushed onto the outer surface of the reactor pressure vessel, transmits and receives an ultrasonic wave to and from a penetration having a welded portion while changing an incident angle of the ultrasonic wave. Based on a result of reception of an echo obtained by the reflection of the ultrasonic wave on the inner surface of the penetration, an inclination angle of the penetration relative to a wall surface of the reactor pressure vessel is measured. A circumferential direction position of the penetration, which corresponds to the inclination angle, is calculated based on the relationship of an inclination angle and a circumferential direction position, which have been calculated in advance. Then, the circumferential direction position can be obtained as information on the inspection position.