  We are preparing for an ultra-high resolution x-ray spectroscopy of kaonic
atoms using an x-ray spectrometer based on an array of superconducting
transition-edge-sensor microcalorimeters developed by NIST. The instrument has
excellent energy resolutions of 2 - 3 eV (FWHM) at 6 keV and a large collecting
area of about 20 mm^2. This will open new door to investigate kaon-nucleus
strong interaction and provide new accurate charged-kaon mass value.
