An internal state detector of detecting an internal state of a plate-shape object constituted by a fiber-reinforced resin material that is configured to detect an internal state of the object by using ultrasonic vibrations of Lamb waves, the Lamb waves having, as modes of propagation to the object, a plurality of symmetric modes and a plurality of asymmetric modes in which a propagation velocity varies depending on a frequency, the internal state detector comprising: an exciter that is configured to apply, to a surface of the plate-shaped object including a fiber-reinforced resin, ultrasonic vibrations of the Lamb waves; a vibration detector that is configured to detect ultrasonic vibrations of the Lamb waves, the ultrasonic vibrations being applied by the exciter to the object and propagating in the object; and an internal state estimation calculator that is configured to estimate an internal state of the object, based on a detection result of the vibration detector, wherein the internal state estimation calculator is configured to obtain a propagation time of ultrasonic vibrations propagating in the object corresponding to a frequency, based on an output value of the vibration detector obtained in vibration application by the exciter, to estimate an internal state of the object, based on a delay time of the propagation time relative to a reference propagation time of the object in a frequency range where only the fundamental wave symmetric mode and the fundamental wave asymmetric mode appear, and to output the estimated internal state, the internal state detector comprises a memory that is configured to store reference propagation times of an object whose internal state is known corresponding to a plurality of frequencies in the frequency range where only the fundamental wave symmetric mode and the fundamental wave asymmetric mode appear, the reference propagation times are propagation times for a material which has the same size and the same configuration as those of the object and whose entire portion is a healthy portion, and the internal state estimation calculator is configured to estimate an internal state of the object, based on a reference propagation time of the object corresponding to at least one frequency stored in the memory in the frequency range and based on the propagation time corresponding to a frequency equal to the at least one frequency, and to output the estimated internal state, the internal state estimation calculator being configured to estimate the internal state of the object, based on a maximum time difference in delay times of the propagation time relative to the reference propagation time of the object for each frequency of the plurality of frequencies in the frequency range where only the fundamental wave symmetric mode and a fundamental wave asymmetric mode appear, and to output the estimated internal state. an exciter that is configured to apply, to a surface of the plate-shaped object including a fiber-reinforced resin, ultrasonic vibrations of the Lamb waves; a vibration detector that is configured to detect ultrasonic vibrations of the Lamb waves, the ultrasonic vibrations being applied by the exciter to the object and propagating in the object; and an internal state estimation calculator that is configured to estimate an internal state of the object, based on a detection result of the vibration detector, wherein the internal state estimation calculator is configured to obtain a propagation time of ultrasonic vibrations propagating in the object corresponding to a frequency, based on an output value of the vibration detector obtained in vibration application by the exciter, to estimate an internal state of the object, based on a delay time of the propagation time relative to a reference propagation time of the object in a frequency range where only the fundamental wave symmetric mode and the fundamental wave asymmetric mode appear, and to output the estimated internal state, the internal state detector comprises a memory that is configured to store reference propagation times of an object whose internal state is known corresponding to a plurality of frequencies in the frequency range where only the fundamental wave symmetric mode and the fundamental wave asymmetric mode appear, the reference propagation times are propagation times for a material which has the same size and the same configuration as those of the object and whose entire portion is a healthy portion, and the internal state estimation calculator is configured to estimate an internal state of the object, based on a reference propagation time of the object corresponding to at least one frequency stored in the memory in the frequency range and based on the propagation time corresponding to a frequency equal to the at least one frequency, and to output the estimated internal state, the internal state estimation calculator being configured to estimate the internal state of the object, based on a maximum time difference in delay times of the propagation time relative to the reference propagation time of the object for each frequency of the plurality of frequencies in the frequency range where only the fundamental wave symmetric mode and a fundamental wave asymmetric mode appear, and to output the estimated internal state.