  Improved statistics and mass-composition-sensitive observation are required
to clarify the origin of ultra-high energy cosmic rays (UHECRs). Inevitably in
the future, the UHECR observatories will have to be expanded due to the small
flux; however, the upgrade will be expensive with the detectors currently in
use. Hence, we are developing a new fluorescence detector for UHECR
observation. The proposed fluorescence detector, called cosmic ray air
fluorescence Fresnel-lens telescope (CRAFFT), has an extremely simple structure
and can observe the longitudinal development of an air shower. Furthermore,
CRAFFT has the potential to significantly reduce costs for the realization of a
huge observatory for UHECR research. We deployed four CRAFFT detectors at the
Telescope Array site and conducted the test observation. We have successfully
observed ten air-shower events using CRAFFT. Thus, CRAFFT can be a solution to
realize the next generation of UHECR observatories.
