1. An image processing device for detecting a lesion candidate region from a medical image, comprising: parameter setting means for setting a parameter to be used for detecting the lesion candidate region; and lesion candidate region detecting means for assessing the medical image using the parameter set by the parameter setting means and detecting the lesion candidate region based on a result of the assessment, wherein the parameter setting means includes parameter inputting means configured to input a first parameter indicating dimension of the lesion, and second parameter calculating means configured to calculate a second parameter based on the first parameter indicating the dimension of the lesion, and wherein the lesion candidate region detecting means includes lesion candidate region extracting means configured to (i) calculate a feature amount indicating a shape of an organ surface, by using the second parameter in the medical image and (ii) extract the lesion candidate region based on the feature amount calculated based on the second parameter which in turn was calculated based on the first parameter indicating the dimension of the lesion.