Pipe-wall thinning due to corrosion is an essential problem for piping systems in nuclear power plants. This study aims to characterize periodic rough surfaces on the inside thinned walls of pipes. This paper focuses on the frequency spectrum of the reflected waves to investigate the reflection of ultrasonic waves from periodic rough surfaces. The spectrum dropped at specific frequencies by using block-shaped specimens with parallel flaws. The tendency was observed in both the experiment and simulation. The results indicated that the wavelength calculated from the frequency of amplitude drop roughly corresponded to the flaws’ pitch.