An improved radio-wave insulating device for a high frequency signal distributor, comprising a cover 2a and a case body 3a. The case body 3a is provided with a plurality of protuberances 32a which slightly outsizes the corresponding notches 22a of the cover 2a. Thus when the cover 2a is mounted in place in the case body 3a, a force, which results from the deformation of the protuberances 32a in the notches 22a, is provided parallel to the corresponding side of the cover, thus ensuring a firm engagement between the cover and the case body, without causing the bulging-out of the case body.