A method of estimating the propagation delay between two remote sites using a wireless communication module that is compatible with IEEE 802.15.4g is proposed. In our previous paper, we reported that propagation delay due to water vapor can be estimated using digital terrestrial broadcasting waves, which is a passive method from the user's view point. In this paper, we propose an active method that allows the propagation delay between two remote sites of the user's choice to be monitored, as long as a line of sight exists. A real-time propagation delay monitoring system with wireless communication modules, (wireless two-way interferometry [Wi-Wi] modules,) is developed and tested. The data obtained using Wi-Wi modules separated by 4.25 km show about 14-cm variation of propagation delay in 1 day, in good agreement with values obtained by ground-based meteorological observation. This study shows that this technique enables measurement of the surface propagation delay in the horizontal direction with millimeter precision and high spatial and temporal resolution at a low cost.