A plant monitoring apparatus 1 includes: an extraction unit 2 that extracts a feature amount in a frequency response of vibration of a target plant with use of the vibration; a calculation unit 3 that calculates a change that indicates growth of the plant, based on the extracted feature amount and a reference feature amount that corresponds to a reference state of the plant; and an estimation unit 4 that estimates a state of the plant by, with use of the calculated change, referencing state information in which changes of the feature amount from the reference feature amount corresponding to growth of the plant are associated with states of the plant.