1. An information filtering apparatus for collecting and filtering information relating to a target theme, comprising: an exhaustivity calculation unit comprising hardware of a computing device and configured to calculate exhaustivity as a ratio of a range of a user-acquired information group in a range of a target theme-related information group relating to the target theme, to the range of the target theme-related information group in a vocabulary space formed of a collection of words representing a ratio of a user-acquired part to an entire part of the target theme, based on the number of words contained in the range of the target theme-related information group and the number of words contained in the range of the user-acquired information group; a sending unit comprising hardware of the computing device and configured to send the exhaustivity calculated by the exhaustivity calculation unit to a client terminal; and a theme space construction unit comprising the hardware and configured to generate a search keyword set for searching for information relating to the target theme by using a “user-acquired” history based upon a feedback of information determined to have been acquired by a user, collect information by use of the search keyword set, and extract words relating to the target theme from the collected information to construct the range of the target theme-related information group, wherein the theme space construction unit has a unit that calculates importance of words in terms of the target theme by using TF-IDF and a thesaurus, and extracts the words relating to the target theme in consideration of the importance.