Disclosed is a method for diagnosing the growth of a crop with less influence of the weather without needing measurement and detection of the ground surface. The method is provided with a step (S1) of irradiating stalks and leaves of a crop with laser pulses from irradiation points above the stalks and leaves of the crop, a step (S2) of receiving the laser pulses reflected by the stalks and leaves of the crop or the ground surface at a light-receiving point, a step (S3) of acquiring data including the distances from the irradiation points to the reflection points and relating to a group of three-dimensional points of the stalks and leaves of the crop by measuring the propagation times from the irradiation with the laser pulses to the light receptions, a step (S4) of calculating the plant canopy position of the stalks and leaves of the crop on the basis of the data relating to the group of three-dimensional points, a step (S5) of calculating the laser pulse entrance depths from the plant canopy position to the reflection points and the laser pulse entrance rate from the plant canopy position and the data relating to the group of three-dimensional points, and a step (S6) of estimating the plant cover ratio on the basis of the laser pulse entrance depths and the laser pulse entrance rates.
