To evaluate the indoor measurement accuracy of multi-junction solar cells and modules experimentally, we have successfully used natural sunlight whose spectral irradiance is close to the reference sunlight. The reference sunlight for evaluating the performance of solar cells can be realized by choosing natural sunlight incident on a sun-facing 37 degrees tilted surface at air mass of 1.5, when the atmosphere has turbidity of 0.27, precipitable water of 1.42 cm and so on. The measurement of atmospheric parameters was performed based mainly on the measurement of direct solar spectral irradiance and additionally on the measurements of irradiance and illuminance. In addition, measured values were compared with aerological observation data. The comparison of I-V characteristics measured outdoors and indoors shows that our pulsed-light solar simulator is capable of measuring multi-junction amorphous silicon alloy solar cells and modules accurately enough when its irradiance is set by an amorphous silicon reference cell. This fact is also confirmed by the examination of spectral irradiance on the test plane of the solar simulator. Moreover, we showed that irradiance (or illuminance) can be used instead of atmospheric turbidity and also water vapor content on a surface of the Earth can be used as a rough estimate of precipitable water for finding the reference sunlight conditions.