Provided is a device that calculates changes in the effect of vegetation on the stability of a slope. This vegetation effect calculation device comprises at least an I/O unit, a correction value calculation unit, and a correction unit. The I/O unit obtains: the moisture content of a slope; the relationship between the moisture content of the slope and the adhesion of the soil; the optimum moisture content ratio for the slope; and the vegetation adhesion being the adhesion of vegetation growing abundantly on the slope. A vegetation effect calculation unit uses the obtained moisture content of the slope and the relationship between the obtained moisture content and the soil adhesion and calculates a correction value. The correction unit uses the obtained vegetation adhesion and the calculated correction value and calculates a corrected vegetation adhesion.
