An object of the present invention is to provide a housing block retaining construction which allows a release tool to be easily operated, and will not interfere with the withdrawal of a housing block. According to the present invention, there is provided that a housing block retaining construction comprises a retaining lance slanting in the insertion direction provided in an insertion opening of a housing holder, a notch in which a distal end of a release tool is engageable formed in a distal end of the retaining lance, a retaining projection for engagement with the retaining lance provided on a housing block, and a recess for allowing the release tool to pass therethrough formed in that portion of the retaining projection corresponding to the notch.