A comprehensive theory of multi-chamber air infiltration measurement using a single tracer gas is introduced from the general stand point of system identification. The thermal network model can be applied not only to the temperature transfer and diffusion system but also to the tracer gas transfer system. This model is formulated mathematically in a stale equation. The coefficents in the state equation represent airflow rates of infiltration. Two theories for estimating these coefficients are deduced from the least square. These are batch and successive identification. Further, the evalualion method of estimation errors that has been so far insufficient is described. Finally, the accuracy and the practicality of the theory are confirmed by the actual measurement system and experiments.