To make the cover made of a synthetic resin available for an LNB (low-noise frequency converter) by suppressing the secondary radiation of electromagnetic waves having a local oscillation frequency by short circuiting the chassis and electromagnetic shielding lid of the LNB to each other by inserting a metallic plate between the chassis and cap.

After an LNB circuit board is set in a chassis 1, a box-like electromagnetic shielding lid 2, having an opened bottom is inserted in the opening of the top face of the chassis 1, and the lid 2 is fixed to the chassis 1 through the LNB circuit board by tightening screws 3. The secondary radiation of electromagnetic waves having a local oscillation frequency due to the gap between the chassis 1 and lid 2 is prevented by attaching an elastic metallic plate 5 carrying recessing and projecting sections, formed in the vertical direction (thickness direction) to the outer periphery of the lid 2, so as to short-circuit the chassis 1 to the lid 2. An LNB cover 6 is made of a synthetic resin for preventing the intrusion of rain water into an LNB. The 4, 7, and 8 of the figure are a frequency-adjusting screw, a waveguide junction, and a connecting section for sending signals down-converted by means of the LNB to a satellite broadcast receiver, etc., through a coaxial cable, respectively.