A transmission probe for transmitting guided waves propagating in the longitudinal direction of a long member and a reception probe for receiving guided waves derived from the guided waves reflected from a predetermined portion of the long member are set on the long member. The guided waves received by the reception probe include a guided wave serving as a second signal that is noise of a desired first signal. The guided wave serving as the second signal having nodes in a circumferential direction distribution of displacement in a specific direction in the circumferential surface of the long member, and the guided waves transmitted by the transmission probe are formed such that the displacement of the guided wave serving as the second signal in the specific direction becomes zero at a specific circumferential surface position of the long member. A probe setting method comprising the steps of: setting the transmission probe for transmitting the guided waves on the circumferential surface of the long member; and setting the reception probe at a position at which the displacement of the guided wave serving as the second signal in the specific direction becomes zero on the circumferential surface of the long member.