1. A similar image providing device comprising: a lesion region extracting unit that extracts a lesion region from a subject diagnostic image; a local image feature extracting unit that divides the lesion region into local images, and extracts local image features that characterize the respective local images; a quantizing unit that quantizes the local image features and outputs quantized data; a lesion classifying unit that, on the basis of the quantized data outputted from the quantizing unit, classifies a lesion; a storing unit storing, per type of lesion, correlation coefficients between local image features and topic variables expressing degrees of progression or degrees of seriousness of lesions; an expected value estimating unit that acquires, from the storing unit, correlation coefficients between topic variables and respective local image features corresponding to the lesion classified by the lesion classifying unit and that are extracted by the local image feature extracting unit, and, by using the acquired correlation coefficients, estimates, for each topic variable, the expected value of the probability that the topic variable generated the local feature within the lesion; an image storing unit that stores, in association, diagnostic images and expected values of topic variable probabilities that are computed in advance for the diagnostic images; and a providing unit that provides, as similar images and from among the diagnostic images stored in the image storing unit, diagnostic images corresponding to expected values of topic probabilities that best approximate the expected values of the topic probabilities that are estimated by the expected value estimating unit.