A system for damage diagnosis for diagnosing a damage that occurred on or within an object, the system comprising: an oscillator for applying a broadband ultrasonic oscillation to the object to generate a broadband Lamb wave within the object; an oscillation detection sensor for detecting the broadband Lamb wave from the object, the detected broadband Lamb wave including at least one mode of Lamb wave; and a processing unit, being connected to the oscillator and the oscillation detection sensor, for obtaining time-frequency transformation data by performing a time-frequency transformation to the broadband Lamb wave detected by the oscillation detection sensor, wherein the time-frequency transformation data indicates a propagation time of the at least one mode of Lamb wave, and the propagation time is the time for Lamb wave to propagate from the oscillator through the oscillation detection sensor, and identifying, based on the propagation time of the at least one mode of Lamb wave in the time-frequency transformation data, whether or not the damage has occurred on or within the object, and/or the extent of the damage that occurred on or within the object, wherein the at least one mode Lamb wave includes a plurality of waves having mutually different frequencies, the processing unit, in the identifying process: calculates propagation times of two of the plurality of waves; calculates a change ratio of the propagation times by means of dividing a difference of the two propagation times by a difference of the frequencies of the two waves; and based on whether or not the change ratio matches a reference value, which is a change ratio for a structure for which a state of damage is known, identifies whether or not the damage has occurred on or within the object, and/or identifies the extent of the damage that occurred on or within the object, and the at least one mode is A1 mode. an oscillator for applying a broadband ultrasonic oscillation to the object to generate a broadband Lamb wave within the object; an oscillation detection sensor for detecting the broadband Lamb wave from the object, the detected broadband Lamb wave including at least one mode of Lamb wave; and a processing unit, being connected to the oscillator and the oscillation detection sensor, for obtaining time-frequency transformation data by performing a time-frequency transformation to the broadband Lamb wave detected by the oscillation detection sensor, wherein the time-frequency transformation data indicates a propagation time of the at least one mode of Lamb wave, and the propagation time is the time for Lamb wave to propagate from the oscillator through the oscillation detection sensor, and identifying, based on the propagation time of the at least one mode of Lamb wave in the time-frequency transformation data, whether or not the damage has occurred on or within the object, and/or the extent of the damage that occurred on or within the object, obtaining time-frequency transformation data by performing a time-frequency transformation to the broadband Lamb wave detected by the oscillation detection sensor, wherein the time-frequency transformation data indicates a propagation time of the at least one mode of Lamb wave, and the propagation time is the time for Lamb wave to propagate from the oscillator through the oscillation detection sensor, and identifying, based on the propagation time of the at least one mode of Lamb wave in the time-frequency transformation data, whether or not the damage has occurred on or within the object, and/or the extent of the damage that occurred on or within the object, wherein the at least one mode Lamb wave includes a plurality of waves having mutually different frequencies, the processing unit, in the identifying process: calculates propagation times of two of the plurality of waves; calculates a change ratio of the propagation times by means of dividing a difference of the two propagation times by a difference of the frequencies of the two waves; and based on whether or not the change ratio matches a reference value, which is a change ratio for a structure for which a state of damage is known, identifies whether or not the damage has occurred on or within the object, and/or identifies the extent of the damage that occurred on or within the object, and the at least one mode is A1 mode.