An ultrasonic flaw detection method comprising: generating ultrasound waves corresponding to transmission signals to be provided to an ultrasonic probe and sending out the ultrasound waves into a specimen, and receiving by the ultrasonic probe echoes of the respective ultrasound waves having propagated through the specimen and outputting the received echoes as reception signals; generating, as the transmission signals, signals that are used by the ultrasonic probe to send out the ultrasound waves to the specimen obliquely at a respective plurality of angles; and determining, from the reception signals, amplitudes (E1, E2) of the echoes corresponding to the plurality of angles, and periods of time from when the respective ultrasound waves are sent out until the respective echoes are received, as reception times (T1, T2), and identifying a location of an acoustic discontinuous portion in the specimen given by a flaw by estimating a distance from the probe to the flaw from the reception times (T1, T2), said method characterized by identifying the location of the flaw from a ratio between the amplitudes (E1, E2) and the reception times (T1, T2) by determining on which of a front side and a back side of the specimen the flaw is located using differences between changes in echo height relative to the distance between an AOSO mode and an S1 mode of the ultrasound waves; configuring the frequency and the angle of incidence of said ultrasound waves to first respective values so that first plate waves of A0 mode and S0 mode propagate through said specimen at the same, first phase velocity; and configuring the frequency and the angle of incidence of said ultrasound waves to second respective values so that second plate waves of S1 mode propagate through the specimen at a second, different phase velocity, said AOSO mode being a mode having characteristics of both said A0 mode and said S0 mode. generating ultrasound waves corresponding to transmission signals to be provided to an ultrasonic probe and sending out the ultrasound waves into a specimen, and receiving by the ultrasonic probe echoes of the respective ultrasound waves having propagated through the specimen and outputting the received echoes as reception signals; generating, as the transmission signals, signals that are used by the ultrasonic probe to send out the ultrasound waves to the specimen obliquely at a respective plurality of angles; and determining, from the reception signals, amplitudes (E1, E2) of the echoes corresponding to the plurality of angles, and periods of time from when the respective ultrasound waves are sent out until the respective echoes are received, as reception times (T1, T2), and identifying a location of an acoustic discontinuous portion in the specimen given by a flaw by estimating a distance from the probe to the flaw from the reception times (T1, T2), said method characterized by identifying the location of the flaw from a ratio between the amplitudes (E1, E2) and the reception times (T1, T2) by determining on which of a front side and a back side of the specimen the flaw is located using differences between changes in echo height relative to the distance between an AOSO mode and an S1 mode of the ultrasound waves; configuring the frequency and the angle of incidence of said ultrasound waves to first respective values so that first plate waves of A0 mode and S0 mode propagate through said specimen at the same, first phase velocity; and configuring the frequency and the angle of incidence of said ultrasound waves to second respective values so that second plate waves of S1 mode propagate through the specimen at a second, different phase velocity, said AOSO mode being a mode having characteristics of both said A0 mode and said S0 mode.