In non-destructive inspection, it is very important to know in-pipe corrosions form because its information is available to infer kind of corrosions and cause of corrosions. An ultrasonic flaw detection method is often used for because of easy operation. However measurement accuracy is not good. Especially it is very difficult to get accurate thickness information at complex corrosions area where there are multiple corrosions. In this paper, we proposed the algorithm to improve a measurement precision of ultrasonic flaw detection method by measuring multipoints data and frequency characteristics of ultrasonic wave signals. The wavelet analysis of ultrasonic echo is used in order to detect candidates of reflect wave from a crrosion. We applied the algorithm to a pipe with the natural corrosions which have complex form and showed the utility compared to thickness measurement method.