An apparatus state detector and its associates are provided to save time and effort for learning combinations of operation states of all apparatuses at home and commit fewer estimation errors even when an unknown apparatus starts operating. The apparatus state detector includes measuring means that measures a physical quantity of an environment in which an apparatus is placed, feature-quantity calculation means that calculates a feature quantity of the measured value measured by the measuring means, storage means that stores in advance the feature quantity of each apparatus and an apparatus state associated with the feature quantity in a reference-apparatus entry dictionary, and apparatus-state detection means that searches the reference-apparatus entry dictionary for a feature quantity by using a feature quantity calculated by the feature-quantity calculation means as a search key and detects an apparatus state based on the apparatus state associated with the retrieved feature quantity.