PURPOSE: To obtain an ultrasonic flaw detecting device which can detect flaws stably over the entire length of a material to be inspected without positioning of the material to be inspected and without measuring the length of the material to be inspected.

CONSTITUTION: This ultrasonic flaw detecting device is provided with an aerial ultrasonic sensor 16 which is mounted on a holding mechanism 4 for ultrasonic flaw detection by moving along on a material 1 to be inspected in the shape of a rotating pipe or rod and sends and receives an ultrasonic wave, a sending and receiving circuit 17 for sending a transmit pulse to the aerial ultrasonic sensor 16 and receiving a reflection echo received by the aerial ultrasonic sensor 16, a gate circuit 18 for judging presence of the reflection echo received by the aerial ultrasonic sensor 16 and a judgment value setting circuit 19 for setting a judgment value at the gate circuit 18, a front end and a rear end of the material 1 to be inspected is detected by the aerial ultrasonic sensor 16 and driving of a carriage 5 and bringing into contact with and separating from the material of the holding mechanism 1 is controlled by this.