PURPOSE: To elevate the reliability and maintaining and servicing property by shifting vibrators on the odd-numbered stages from those on the even-numbered stages heightwise in the array of vibrators based on the frequency used at the lower limit as reference to realize the RDT directivity with a less number of vibrators.

CONSTITUTION: Vibrators 1 and 2 of 48 pieces as electroacoustic conversion elements are arrayed circumferentially in 8 stages heightwise while vibrators 1 on the odd-numbered stages are shifted heightwise by 3.75° equivalent to π/m in terms of circumferential angle from those 2 on the even-numbered stages. This can lower the production cost of a transmitter/receiver substantially while elevating the reliability and maintaining and servicing property.