PURPOSE: To enhance detecting accuracy, by tilting an ultrasonic wave receiving element, whose relative position relation is changed with a material to be measured, at a specified angle with respect to the central axis of the material to be measured.

CONSTITUTION: Relative position relation of a manipulator 72 with a material to be measured (not shown) is changed by a manipulator control device 71. An ultrasonic wave transducer 73, which is used for sending and receiving the waves, is provided on the manipulator 72. The received wave signal is stored in a memory 76 through a received signal amplifier 74 and an A/D converter 75. A data processing device 70 is constituted of an interface control unit 77 and the like. An oscillator 80 for the transducer 73, the device 71 and the memory 76 are controlled by the device 70. An angle θ is obtained by θ=sin1.22λ/D(rad), where D is the diameter of the transducer 73 and λ is the wavelength of the ultrasonic wave. The transducer 73 is tilted by the angle θ with respect to the central axis of the material to be measured so as to obtain the maximum sensitivity.