PURPOSE: To detect the position of flaw at a high accuracy by statistic processing of a reflected echo train with a computer after it is sampled being shifted by unit time with a sampling circuit.

CONSTITUTION: A high frequency sine wave outputted from a frequency totalizer 6 is divided down to a desired frequency with a frequency divider circuit 7, an output of which drives a pulse generation circuit 9 as gate signal to apply a high voltage pulse to a transducer 10. Echos reflected from flaw in an object to be inspected are converted into electrical signal again with the transducer 10 and inputted into a sampling circuit 13 passing through an amplifier 12. A sampling trigger pulse is applied to the sampling circuit from a delay circuit 8 being shifted by a unit time starting from the pulse delayed by a delay time set with a computer 15 with the rising of a transmission pulse as reference. A sampling output is inputted into the computer 15 after an A/D conversion for statistic processing and the position of flaw is displayed as image on a CRT.