An ultrasonic flaw detection method for detecting a flaw on an inner surface of a metallic pipe using ultrasonic waves, the ultrasonic flaw detection method comprising: waveform holding step for acquiring and holding waveform data of an echo signal when an ultrasonic probe that generates ultrasonic signals toward the inner surface and the metallic pipe are moved relative to each other; signal analyzing step (S102, S103) for calculating a path length up to receiving an echo signal from the inner surface and a change rate of the path length based on the waveform data held, wherein the beam path length is obtained from a time when a surface echo reflected from an outer surface of said metallic pipe is received until a bottom echo reflected from said inner surface is received, and wherein the change rate of the path length is with respect to an amount of said relative moving of said ultrasonic probe and said metallic pipe; and flaw detecting step for detecting a flaw on the inner surface based on the path length and the change rate of the path length, wherein the signal analyzing step further calculates a height of the echo signal from the inner surface based on the waveform data held, and wherein the flaw detecting step determines that a flaw is present in a portion satisfying a first condition in which the path length is equal to or smaller than a given path length threshold, with respect to a portion not satisfying the first condition, the flaw detecting step determines that a flaw is present in the portion not satisfying the first condition when the portion not satisfying the first condition satisfies a second condition in which the change rate of the path length is equal to or greater than a given path-length change rate threshold and the height of the echo signal from the inner surface within a search range preset with respect to a position of the change rate of the path length being equal to or greater than the given path-length change rate threshold as a reference position is equal to or smaller than a given height threshold (DA, S407), and the flaw detecting step determines that no flaw is present in a portion not satisfying both of the first and second condition. waveform holding step for acquiring and holding waveform data of an echo signal when an ultrasonic probe that generates ultrasonic signals toward the inner surface and the metallic pipe are moved relative to each other; signal analyzing step (S102, S103) for calculating a path length up to receiving an echo signal from the inner surface and a change rate of the path length based on the waveform data held, wherein the beam path length is obtained from a time when a surface echo reflected from an outer surface of said metallic pipe is received until a bottom echo reflected from said inner surface is received, and wherein the change rate of the path length is with respect to an amount of said relative moving of said ultrasonic probe and said metallic pipe; and flaw detecting step for detecting a flaw on the inner surface based on the path length and the change rate of the path length, wherein the signal analyzing step further calculates a height of the echo signal from the inner surface based on the waveform data held, and wherein the flaw detecting step determines that a flaw is present in a portion satisfying a first condition in which the path length is equal to or smaller than a given path length threshold, with respect to a portion not satisfying the first condition, the flaw detecting step determines that a flaw is present in the portion not satisfying the first condition when the portion not satisfying the first condition satisfies a second condition in which the change rate of the path length is equal to or greater than a given path-length change rate threshold and the height of the echo signal from the inner surface within a search range preset with respect to a position of the change rate of the path length being equal to or greater than the given path-length change rate threshold as a reference position is equal to or smaller than a given height threshold (DA, S407), and the flaw detecting step determines that no flaw is present in a portion not satisfying both of the first and second condition.