To provide an outside face corrosion measurement method, capable of identifying both of the position and the dimension of local outside face corrosion in a rod-shaped member by means of ultrasonic flaw detection.

An ultrasonic wave is radiated from a probe 4 to one end face of the rod-shaped member 1, and a reflected wave echo A, which is reflected on an interface with the outer circumferential face of the rod-shaped member inside and returned before a bottom face wave echo B from the other end face, is observed. On the basis of positional information and waveform distribution information or the time base of the reflected wave echo, the position and the area of local outside face corrosion 2 in the rod-shaped member are measured.