A plant monitoring apparatus comprising: a model generating means for executing a growth model simulation on a target plant to generate a growth model with respect to a portion of the target plant by i) modeling a target plant to generate one or more plant models with respect to respective one or more states of the portion, ii) virtually applying vibration to the plant models to generate vibration information of each of the plant models in a time domain, iii) converting the vibration information into a frequency domain to generate a frequency response per plant model, iv) calculating a first feature amount per plant model based on the generated frequency response, v) calculating a first difference, per plant model, between the first feature amount of the plant model and a reference feature amount, and vi) generating a first growth model for each plant model by associating information identifying a state of the portion corresponding to said plant model, a plurality of growth amounts of each identified state of the portion, and the calculated first differences corresponding to the plurality of growth amounts, the reference feature amount being a feature amount in a reference state of the target plant; a collecting means for collecting, from a vibrometer, vibration information in a time domain which has been generated by applying vibration to the portion of the target plant; an extracting means for converting the vibration information in the time domain into a frequency domain to generate a frequency response, and extracting a second feature amount from the generated frequency response; a calculating means for calculating a second difference between the extracted second feature amount and the reference feature amount; and an estimating means for estimating a growth amount of the target plant by referencing the growth model, selecting a first difference, among the plurality of first differences of each of the states, that is similar to the extracted second difference, acquiring a growth amount associated with the selected first difference, wherein each of the reference feature amount, the first feature amount, and the second feature amount have at least a resonance frequency and a Q value representing a sharpness of the resonance frequency. a model generating means for executing a growth model simulation on a target plant to generate a growth model with respect to a portion of the target plant by i) modeling a target plant to generate one or more plant models with respect to respective one or more states of the portion, ii) virtually applying vibration to the plant models to generate vibration information of each of the plant models in a time domain, iii) converting the vibration information into a frequency domain to generate a frequency response per plant model, iv) calculating a first feature amount per plant model based on the generated frequency response, v) calculating a first difference, per plant model, between the first feature amount of the plant model and a reference feature amount, and vi) generating a first growth model for each plant model by associating information identifying a state of the portion corresponding to said plant model, a plurality of growth amounts of each identified state of the portion, and the calculated first differences corresponding to the plurality of growth amounts, the reference feature amount being a feature amount in a reference state of the target plant; a collecting means for collecting, from a vibrometer, vibration information in a time domain which has been generated by applying vibration to the portion of the target plant; an extracting means for converting the vibration information in the time domain into a frequency domain to generate a frequency response, and extracting a second feature amount from the generated frequency response; a calculating means for calculating a second difference between the extracted second feature amount and the reference feature amount; and an estimating means for estimating a growth amount of the target plant by referencing the growth model, selecting a first difference, among the plurality of first differences of each of the states, that is similar to the extracted second difference, acquiring a growth amount associated with the selected first difference, wherein each of the reference feature amount, the first feature amount, and the second feature amount have at least a resonance frequency and a Q value representing a sharpness of the resonance frequency.