PURPOSE: To especially facilitate ultrasonic flaw detection from the internal surface of a pressurized water type nuclear power generation plant, by providing a mechanism which rotates a probe adapted to make various specified angles other than 0° in a specified range in the surface facing the inner surface of a pressure container.

CONSTITUTION: When a reactor pressure container RPV1 has an ultrasonic wave incident thereinto horizontally, an ultrasonic wave incident thereinto horizontally, an ultrasonic wave 47 from a probe 32 of 0° is made incident on the position of the point Q vertical to the inner wall of the RPV1. On the other hand, an ultrasonic wave 48 from the probe of 45° is made incident on the inner wall surface P of the RPV1 on a vertical line at the point Q. An ultrasonic wave 48 incident on the wall surface P at an angle θ1 is refracted according to the ratio between the sound velocity under water and that in steel and is incident on the steel at 45° according to the Snell's law to perform a horizontal flaw detection for the RPV1 in a range of AWB. On the other hand, when an ultrasonic wave is made incident on the RPV1 in the vertical way, a rotary drie source 45 is driven to rotate gears 43 and 44, which makes a circular track 42 turn meshed with the gear 43 to rotate a probe holder body 30 in the direction of a' or b' on the basis of a rotation detector 46. Thus, a scanning is performed vertically in the range of A'WB'.