This paper describes analytical results obtained for floor penetration loss characteristics and their frequency dependency by measurements in multiple frequency bands, including those above 6GHz, in an indoor office environment. Measurement and analysis results confirm that the floor penetration loss depends on two dominant components: the transmission path through floors, and the path traveling through the outside building. We also clarify that these dominant paths have different path loss characteristics and frequency dependency. The transmission path through floors rapidly attenuates with large inter-floor offsets and in high frequency bands. On the other hand, the path traveling through outside of the building attenuates monotonically as the frequency increases. Therefore, the transmission path is dominant at short inter-floor offsets and low frequencies, and the path traveling through the outside is dominant at high number of floors or high frequency. Finally, we clarify that the floor penetration loss depends on the frequency dependency of the dominant path on the basis of the path loss characteristics of each dominant path.