A new mask inspection system for 150nm and 130nm semiconductor devices which utilizes a DUV laser of 257nm wavelength for an inspection illumination has been developed. A newly developed optical phase shift disk cancels the speckle noise caused by the high coherency of a laser. The phase shift disk has micro pits with different depths disposed randomly over the entire plate surface. The speckle pattern changes randomly by rotating the plate, and averaging pattern image by Time Delay Integration (TDI) sensor cancels the speckle noise of the laser illumination. Using this method, inspection of masks was realized at DUV wavelength.