PURPOSE: To eliminate the problem of a conventional apparatus in which the operating efficiency is low since, when a welded part and ultrasonic flaw detection near it are obtained by using the conventional apparatus, it is waited until the temperature of the flaw detection position is sufficiently cooled.

CONSTITUTION: An ultrasonic flaw detector 3 is runnable in a separate state from a welding machine 2 on rails 1 laid along the part to be welded of a steel material P for traveling the machine 2 and executable for ultrasonic flaw detection at a welded part W and its vicinity. A distance setting mechanism D is setable at a separate distance between the machine 2 and the detector 3 under the conditions for executing the flaw detection in the state in which the temperature of the part W becomes a critical temperature: A1 or lower of the steel.