PURPOSE: To realize plant state detecting device capable of supporting the determination of a state threshold and determining a proper state threshold.

CONSTITUTION: This invention relates to the improvement of existing plant state detecting device for detecting a plant state. This detecting device is provided with a detection means 2 for inputting an observation value of a plant 1 and detecting the state of the plant 1 based upon a state threshold, a data collecting means 41 for collecting observation values from the plant 1, a frequency distribution preparing means 43a for preparing frequency distribution based on the observation values collected by the means 41, and a threshold determining means 43b for determining a state threshold based upon the frequency distribution prepared by the means 43a and constituted so as to change the state threshold of the means 2 based on the state threshold determined by the means 43b.