Air change rates in a cross-ventilation model were measured from the decay curves of video image signals obtained by the step down method assuming perfect mixing of tracer mists inside the spaces. From the wind tunnel experiments, the following conclusions were drawn. 1) Good correlation between image signals and luminous reflectances of achromatic color chips was achieved by correctly adjusting the pedestal level of the video camera. 2) The ray extinction due to lighting scattering did not affect the measurement accuracy of the air change rates in the two-dimensional model. 3) The tracer mists in a diameter between 0.25μm and 2.0μm could obtain the same measurement accuracy. 4) For oil mists as tracer, laser powers with more than 0.5 watt were enough to visualize the flow patterns in the model. 5) The air change rates were proportional to the reference velocities. 6) Compared to the air change rates obtained by the tracer gas procedure using a high-speed hydrocarbon analyzer, the proposed method could prove the capability of measuring those with an error of less than 9% in the range between 0.25 m/s and 2.5 m/s of reference velocities.