1. A content search device configured to search for similar content data, comprising: a content data storage unit configured to store a plurality of pieces of content data in association with respective content identifiers in a storage device, each of the plurality of pieces of content data including at least one known feature type selected from a group of feature types including an image feature, an acoustic feature and a semantic feature, at least one piece of content data including feature types; feature quantity computing means for computing, for each piece of the content data stored in the content data storage unit, a known feature quantity corresponding to a known feature type included in a target piece of the content data, associating, as associated data sets for each piece of the content data, a content identifier of the target piece of the content data, the known feature type of the target piece of the content data, and the computed known feature quantity to each other, and storing the associated data sets in the storage device as feature quantity data; unknown feature quantity computing means for computing a vector space from the feature quantity data including sets of the known feature quantity, computing, for each piece of the content data, an unknown feature quantity corresponding to an unknown feature type based on correlations between (i) the known feature quantity corresponding to the known feature type included in the target piece of the content data and (ii) the computed vector space, the unknown feature type being different from the known feature type included in the target piece of the content data, and associating, for each piece of the content data, the content identifier of the target piece of the content data, the unknown feature type, and the computed unknown feature quantity to update 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 known feature quantities and the unknown feature quantities stored in the feature quantity data; 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; and a determining unit for determining which, if any, of the content data stored in the content data storage unit does not have each of an image feature, an acoustic feature and a semantic feature stored therewith in the content data storage unit, and wherein the unknown feature quantity computing means computes respective unknown feature quantities for each of the content data stored in the content data storage unit that do not have an image feature, an acoustic feature and a semantic feature stored therewith in the content data storage unit.