In Heterogeneous network, a number of small cells are distributed in conventional macro-cellular environments, in order to improve the communication speed. The interference between outdoor and indoor cells then becomes to be a significant problem, so that its propagation loss characteristics should be evaluated in outdoor- indoor propagation environments. This paper proposes a mathematical expression for representing electric field strength of the path passing through an aperture center, called here ‘Single Aperture Field (SAF) method’, and presents experimental results conducted in an anechoic chamber for confirming the effectiveness of this method.