Abstract: Experiments comparing the measured value of ultrasonic thickness measurements and mechanical measurements on rough corroded materials without flattening the test surface were carried out. An average measurement error of from 0.6 mm to 2.2 mm was found in the ultrasonic measurements. To address the problem, an error correction method using surface echo duration was proposed. A 2.25 MHz broadband type transducer was suitable for the measurement and the proposed method can also be applied to a painted test surface.