Abstract: An automatic ultrasonic flaw detecting system adapted to search flaws on steel pipes being continuously transferred and provide markings at the positions of flaws on the steel pipes. The system comprises a probe adapted to search flaws on the steel pipe, a holder for the probe, and a flaw pulse generation circuit adapted to generate flaw pulse signals in response to the flaw signals from the probe. A travel measurement pulse generator generates measurement pulse signals by measuring the distance which the steel pipes have travelled. A pipe and sensor detects the end of the steel pipe and generates pipe end detection signals. A flaw tracking control apparatus tracks the flaws in response to the flaw pulse signals, the measurement pulse signals and the pipe end detecting signals and generates marking control signals. A marking is provided at the position of flaws detected on the steel pipe in response to the marking control signals.