1. A content search device configured to search for similar content data from different types of content data, comprising: a content data storage unit configured to store a plurality of pieces of content data including at least any one of an image feature, an acoustic feature, and a semantic feature in a storage device in association with respective content identifiers; feature quantity computing means for, for each piece of the content data stored in the content data storage unit, computing a feature quantity of at least any one of the image feature, the acoustic feature and the semantic feature included in the piece of the content data, and for storing, in the storage device, feature quantity data in which the content identifier, the feature type among the image feature, the acoustic feature and the semantic feature, and the feature quantity are associated with each other; unknown feature quantity computing means for computing an unknown feature quantity of each feature type not associated with the content identifier by use of the feature quantity of the feature type associated with the content identifier, and for storing the unknown feature quantity as a feature estimated value in the feature quantity data; distance computing means for computing a distance indicating a similarity between each two pieces of the content data based on the feature quantities and the feature estimated values stored in the feature quantity data; and display means for determining a display position of a thumbnail for each piece of the content data based on the distances computed by the distance computing means, and for displaying the thumbnail on a display device.