1. An apparatus for cataloging and retrieving image data obtained by converting scannable documents into image signals, comprising memory means for storing image data, search data for searching image data stored in said memory means and reduced image data obtainable by thinning image data stored in said memory means, display means, input means including a search key which can be operated and released for entering a search command, and control means programmed to check whether said search key is operated, to sequentially display on said display means reduced image data from said memory means while said search key is operated, and to display particular image data from said memory means when said search key is released, said particular image data corresponding to the reduced image data being displayed on said display means when said search key is released.