1. A method of determining a defocus direction of a pattern image of a reticle pattern that is formed on a resist on a substrate, the pattern image being formed by an exposure apparatus for performing exposure of the resist to light by projecting light from the reticle pattern to the resist, the exposed resist being developed to form a resist pattern, the method comprising: an image capturing step of capturing a resist image of the resist pattern to obtain image data, wherein the resist pattern includes a hollow in the resist or a solid of the resist; an extracting step of extracting a feature of the image data to obtain extracted feature data, wherein the extracting step differentiates the image data to produce differential image data, and calculates, as the extracted feature data, a power spectrum of the differential image data; a preparing step of preparing, in advance, plus defocus direction probability space data that is prepared with respect to image data of the resist patterns of which defocus directions are known as plus and minus defocus direction probability space data that is prepared with respect to image data of the resist patterns of which defocus directions are known as minus; a projecting step of projecting the extracted feature data to each of the prepared plus defocus direction probability space data and the prepared minus defocus direction probability space data to obtain probability data, wherein a distance of the extracted feature data from a center of a data group in the prepared plus defocus direction probability space data corresponds to a probability that the pattern image associated with the extracted feature data has a plus defocus direction, and a distance of the extracted feature data from a center of a data group in the prepared minus defocus direction probability space data corresponds to a probability that the pattern image associated with the extracted feature data has a minus defocus direction; and a determining step of determining the defocus direction based on the probability data obtained with respect to each of the prepared plus defocus direction probability space data and the prepared minus defocus direction probability space data.