1. An apparatus for estimating a malignant tumor, the apparatus comprising: a segmentor configured to segment a first medical image of an object into a first region of interest (ROI) including a mass and a second medical image of the object into a second ROI including a mass; an interest region determiner configured to acquire a first matched ROI and a second matched ROI, which is matched to the first matched ROI based on location information of the first ROI and the second ROI, respectively; a feature extractor configured to extract similar features indicating a degree of similarity between the first matched ROI and the second matched ROI, from the first matched ROI and the second matched ROI; and a classifier configured to generate malignant tumor information indicating whether the mass included in the first matched ROI and the second matched ROI is the malignant tumor, based on the extracted similar features, wherein the classifier comprises: a plurality of first classifiers configured to generate probabilities of accuracy of determination that the mass included in the first matched ROI and the second matched ROI is the malignant tumor by analyzing each of the similar features; and a second classifier configured to generate the malignant tumor information by multiplying the probabilities by respective reliabilities of the plurality of first classifiers as weight.