A mirror box type artificial sky for simulating the CIE standard overcast sky has been designed and constructed, while the verification method has been developed and tested. Ratio between the mirror height above the workplane and the room length is 60:133 or 0.451, whereas the ratio between the mirror height above the workplane and the room width is 5:9 or 0.556. Taking the CIE standard overcast sky as reference, indoor illuminance ratios at various elevation angles relative to the zenith is found to be more consistent than luminance ratios. The largest error of illuminance and luminance ratios are respectively 10% and 43%, obtained at 0° elevation angle. Horizontal workplane illuminance values are on average 11,400lx, with illuminance uniformity U₀ and U₁ of respectively 0.92 and 0.86. Based on test results with a building scale model, four out of nine measuring points inside the model have small errors, four have medium error, and one has a large error of 25%. Most of the obtained errors are within the tolerable range of ±21% from the ideal values. Based on the conducted tests, the constructed mirror box type artificial sky is considered appropriate to be utilized for its purpose.