A box-shaped rack forming an electronic device housing has a plurality of guide grooves for guiding upper and lower edges of electronic circuit packages. A plurality of dummy surface plates are pivotably mounted on a front opening portion of the rack and arranged side by side. Each dummy surface plate is biased by a spring, so that when the electronic circuit package is not inserted in the rack, the dummy surface plate is kept in abutment against the inner surface of the rack to shield a part of the front opening portion of the rack. When the electronic circuit package is inserted into the rack, the dummy surface plate is pushed by the electronic circuit package to pivot inward of the rack, and is finally replaced in position by the package surface plate mounted on the electronic circuit package. When the electronic circuit package is removed from the rack, the dummy surface plate is reversely pivoted to restore an original position and again shield the part of the front opening portion of the rack. Accordingly, even when the electronic circuit package is inserted or exchanged during the operation of the electronic device, the occurrence of electromagnetic interference and the reduction in cooling efficiency can be prevented to thereby improve the efficiency of working such as exchanging of the electronic circuit package.