A bonding interface evaluation method for evaluating a joined state of a bonding interface between materials (1a, 1b), comprising: a transmission step of simultaneously transmitting ultrasonic longitudinal wave and transverse wave in a substantially perpendicular direction to the bonding interface; a reception step of receiving a reflection signal of the longitudinal wave transmitted in the transmission step and reflected by the bonding interface and/or a transmission signal of the longitudinal wave transmitted in the transmission step and transmitted through the bonding interface and a reflection signal of the transverse wave transmitted in the transmission step and reflected by the bonding interface and/or a transmission signal of the transverse wave transmitted in the transmission step and transmitted through the bonding interface; and an evaluation step of evaluating a bonding area, an interface opening or a bonding strength of the bonding interface as the joined state of the bonding interface on the basis of a difference between a predetermined physical quantity of the transmission signal of the longitudinal wave and a predetermined physical quantity of the reflection signal of the transverse wave, a difference between a predetermined physical quantity of the reflection signal of the longitudinal wave and a predetermined physical quantity of the transmission signal of the transverse wave or a difference between a predetermined physical quantity of the reflection signal of the longitudinal wave and a predetermined physical quantity of the reflection signal of the transverse wave, out of the signals received in the reception step, the physical quantity being a waveform, an amplitude or a spectrum of the reflection signal or the transmission signal, wherein the longitudinal wave and the transverse wave are transmitted from at least one probe that has a diameter of 10 mm or less in the transmission step, and the reflection signals and/or the transmission signals are received by the probe in the reception step. a transmission step of simultaneously transmitting ultrasonic longitudinal wave and transverse wave in a substantially perpendicular direction to the bonding interface; a reception step of receiving a reflection signal of the longitudinal wave transmitted in the transmission step and reflected by the bonding interface and/or a transmission signal of the longitudinal wave transmitted in the transmission step and transmitted through the bonding interface and a reflection signal of the transverse wave transmitted in the transmission step and reflected by the bonding interface and/or a transmission signal of the transverse wave transmitted in the transmission step and transmitted through the bonding interface; and an evaluation step of evaluating a bonding area, an interface opening or a bonding strength of the bonding interface as the joined state of the bonding interface on the basis of a difference between a predetermined physical quantity of the transmission signal of the longitudinal wave and a predetermined physical quantity of the reflection signal of the transverse wave, a difference between a predetermined physical quantity of the reflection signal of the longitudinal wave and a predetermined physical quantity of the transmission signal of the transverse wave or a difference between a predetermined physical quantity of the reflection signal of the longitudinal wave and a predetermined physical quantity of the reflection signal of the transverse wave, out of the signals received in the reception step, the physical quantity being a waveform, an amplitude or a spectrum of the reflection signal or the transmission signal, wherein the longitudinal wave and the transverse wave are transmitted from at least one probe that has a diameter of 10 mm or less in the transmission step, and the reflection signals and/or the transmission signals are received by the probe in the reception step.