A unit space generating device (141) which generates a unit space for use when diagnosing an operating state of a plant on the basis of a Mahalanobis distance is provided with: a sampling data acquiring unit (141A) which acquires a sampling data group comprising a plurality of state quantities of the plant, measured with a fixed period; an adoption determining unit (141C) which, on the basis of an adoption probability calculated each time the sampling data group is acquired, determines whether the sampling data group is to be adopted as a unit space generation data group serving as the basis for a unit space; a unit space generating unit (141D) which generates unit spaces on the basis of a plurality of the adopted unit space generation data groups; and an output value acquiring unit (141B) which acquires an output value of the plant corresponding to the sampling data group.