A device for a device for preventing the intensity reduction of an optical signal, an optical emission spectrometer, an optical instrument, and a mass spectrometer including the same are provided. The device for preventing the intensity reduction includes a shielding filter which has a mesh structure capable of blocking RF electromagnetic waves radiated from a plasma field for a wafer processing, is installed in the front of an optical window of an optical emission spectrometer for measuring the plasma field from an emission spectrum image of the plasma field, and collects charging particles passing through the mesh.