1. A mask defect inspection method comprising: configuring an optical system to acquire a dark-field image by irradiating an exposure mask with light having a wavelength, the exposure mask containing a defect; positioning the mask at a defocus position spaced away from a just-in-focus position, such that an image of a partial region on the mask at the defocus position contains only a noise signal; acquiring, using the optical system, an image of the partial region; deciding a detection threshold in a desired inspection region on the mask using signal intensities of the acquired image of the partial region and an area ratio between the 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; positioning the mask in a vicinity of the just-in-focus position; 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.