PURPOSE: To increase the rate of operation in a reactor, by means of heightening the reliability in the driving mechanism of a transducer holder in which a large number of transducers are arranged.

CONSTITUTION: A scanning guide 8 is fixed by an axis at the end of the lower part of a rotary housing 7, and a large number of transducers 24 are arranged at an equal distance, by means of setting up a transducer holder 9 in guide thereof. When the upper driving part of the rotary housing 7 may rotate a rotary axis 13, the transducer holder 9 is moved horizontally along a guide rail 19. The transducer holder 9 can scan on the core in a wide ragne by means of being also rotated by the scanning guide 8 by the rotation of the rotary housing 7 which rotates indipendently from the rotary axis 13. And, the opening and the closing a folding arm 16 is enabled easy by means of allowing a drawing up rod 14 to be moved up and down. A signal cable 25 can be followuped, because the operating stroke of transducer holder 9 is allowed to be shortened.