1. A mask defect inspection method for inspecting the presence/absence of a defect of an exposure mask, comprising: preparing an optical system which acquires a dark-field image by irradiating the exposure mask with light of an arbitrary wavelength; allocating an arbitrary partial region where a uniform dark-field image is obtained on the mask at a defocus position distant from a just-in-focus position, and acquiring an image of the partial region, using the optical system; deciding a detection threshold using signal intensities of the acquired image of the partial region and an area ratio between a desired inspection region and the partial region, so that a signal count indicating signal intensities not less than the detection threshold in the desired inspection region is not more than a target false detection count; allocating the mask in the vicinity of the just-in-focus position, and acquiring an image of the desired inspection region using the optical system; and determining, as a defect, a signal having a signal intensity of the acquired image of the desired inspection region, which indicates an intensity not less than the detection threshold.