To prevent an ultrasonic-wave transmitting medium liquid from directly wetting the inner surfaces of a tube and to reduce the amount of use of the ultrasonic- wave transmitting medium liquid when ultrasonic measurement is a performed from the inner surface side of the tube.

When wall thickness measurement and flaw detection are performed on a tube 3 from the inner surface side of the tube 3 through the use of an ultrasonic sensor 1, the gap between the ultrasonic sensor 1 and tube 3 is filled with the ultrasonic transmitting medium liquid 8. The ultrasonic transmitting medium liquid 8 is sealed in a balloon-shaped bag 2. The ultrasonic sensor 1 is housed together with pigs for centering 4 and 5 in the balloon-shaped bag 2 and supported at the center location of the axis of the tube 3 by the pigs for centering 4 and 5. As it is satisfactory that only the periphery of the ultrasonic sensor 1 is filled with the ultrasonic transmitting medium liquid 8, it is possible to reduce the capacity of the balloon-shaped bag 2 and to shorten the work and time required for ultrasonic measurement. Even if the balloon-shaped bag 2 is broken, as the amount of leakage of the ultrasonic transmitting medium liquid 8 is small, it is possible to reduce the extent of damage.