While a transmitting transducer (2a) for transmitting an ultrasonic wave and a receiving transducer (2b) for receiving an ultrasonic wave are moved within a predetermined circular region (7) on a surface of a material being measured, ultrasonic waves are transmitted and received 10,000 times. Then, arithmetic averaging is performed every time an ultrasonic wave is received, on the ultrasonic wave and ultrasonic waves that have been received until then. For example, the aforementioned predetermined frequency is given by ((n.+-.(1/2)).times.(10.sup.6.times.v/.DELTA.L))(Hz), where .DELTA.L is a variation in distance between the transmitting transducer and the receiving transducer, v is a transmission velocity of an ultrasonic wave transmitting in a material being detected, and n is a natural number. Consequently, it is possible to detect, with high accuracy, the thickness of a concrete material having a narrow width and a thick thickness, the thickness of the covering of a reinforcing bar and the diameter thereof, the depth of a crack and the like.