The fifth generation (5G) of mobile communication systems is actively being investigated worldwide. Various systems exist when considering the microwave bands after the introduction of the 5G systems. Considering microwave propagation characteristics is crucial to minimize possible interferences among systems. This study focused on the fact that wireless communication devices such as smartphones and wireless LANs are primarily used in various indoors environments. Previously, we evaluated the loss due to the difference between the incident angles of the vertical plane with respect to a window through which radio waves enter from outdoor to indoor (O2I). In the present study, we measured the loss when radio waves enter from O2I through a window in the 0.9/2.3/5.1 GHz band, where many IoT devices are expected to be used. From the data analysis of the measured results, we proposed a simple O2I penetration path-loss model using incident angles in the vertical and horizontal planes.