A diaphragm apparatus of a lens for a CCTV camera includes a diaphragm blade which opens and closes a stop aperture of the diaphragm apparatus; and an ND filter which is attached to the diaphragm blade, the ND filter having a spectral transmittance so that the transmittance of infrared radiation therethrough is substantially identical to, or less than, the transmittance of visible light therethrough, or alternatively, the ND filter having a spectral transmittance which cuts out infrared radiation. The ND filter covers the stop aperture when the diaphragm blade is moved to open the aperture.