We have reported the recent activities for research on very-high-frequency (VHF) sound in Japan. In this paper (part-1), we introduce the background, summary of our investigation and the result of subjective hearing threshold in such VHF region. The issues on high pressure of VHF sounds including ultra sounds in air that cause various subjective effects have become important around all world. Similar problems on VHF sounds, for example, noise annoyance by VHF sounds radiated from rodent repelling devices, were emerging also since about 10 years ago in Japan. The ears project Japan which have been established against such issues in recent years decided to conduct VHF research, measurement and evaluation in Japan. We introduce summary of the ears project Japan and measured results of subjective threshold in VHF region positioned as part of most important research issue.