1. An image diagnostic apparatus, comprising: an acquisition unit configured to acquire data of an image having a first resolution, the data generated by imaging a pathological diagnostic object; a conversion unit configured to convert the resolution of the image into a second resolution which is lower than the first resolution; an extraction unit configured to analyze the image of which resolution has been converted into the second resolution and to extract a region of interest (ROI) of the pathological diagnostic object; a determination unit configured to determine whether or not a characteristic region indicating a predetermined image characteristic to be a reference of pathological diagnosis exists in the ROI extracted by the extraction unit, when the ROI is extracted by the extraction unit; and a creation unit configured to create, when the characteristic region exists, an output image by converting the resolution so that a ratio of an area of the characteristic region in the output image to an area of the output image is at least a predetermined ratio and less than 100%, wherein when the ROI is not extracted from the image of which resolution has been converted into the second resolution by the extraction unit, the conversion unit is further configured to convert the resolution of the image into a third resolution which is less than or equal to the first resolution and higher than the second resolution, and the extraction unit is further configured to analyze the image converted into the third resolution, and to extract the ROI of the diagnostic object.