In order to achieve the absolute uncertainty which is less than 0.05% and the relative stability which is 0.03% per year in 0.4～1.0 μm of solar absolute spectral irradiance observation, the slit aperture area must be measured with high-precision in the development of solar absolute irradiance spectra-radiometer. The flux comparison method is developed by using laser scanning for precise measurement of the slit aperture area. The laser-illuminated integrating sphere system is used as a light source, and the high-precision trap detector is used to receive signals. Small size apertures can be measured without a reference aperture. In the measurement of the entrance area of solar absolute irradiance spectra-radiometer of which the nominal area is 7 mm×0.1 mm, the total relative uncertainty is about 4.5×10－4.