High-Altitude Platform Station (HAPS) is a new mobile communication platform, which can provide mobile communication services such as Long Term Evolution (LTE) or 5th Generation New Radio (5G NR) from stratosphere to terrestrial User Equipments (UEs) directly by utilizing aerial vehicles such as balloon, airplane or airship with radio equipment at the altitude of 20km. It has been attracting much attention for its ultra-wide coverage as wide as 50km to 100km in radius and disaster-resilient networks. The service area of HAPS overlaps that of existing terrestrial mobile network due to its ultra-wide coverage and the terrestrial network can be affected by the interference from a HAPS. Although one of the simplest ways to avoid the interference is to separate spectrum for HAPS and terrestrial network exclusively, it is not desirable from the viewpoint of spectral efficiency. Another approach is co-channel spectrum sharing, which enables higher spectrum reuse. However, conventional co-channel spectrum_sharing techniques introduced in LTE or LTE-Advanced require muting data transmission partially in order to mitigate the interference. If the conventional techniques are applied to the HAPS, it leads to a decrease in available radio resource of the HAPS. This paper proposes to apply a co-channel downlink spectrum sharing between HAPS and terrestrial networks without muting HAPS’ transmission. We also make an outdoor propagation measurement to clarify the received signal power from both HAPS and the terrestrial networks in real outdoor fields and show the feasibility of the proposed co-channel spectrum sharing.