  A new method to determine the CO-to-H$_2$ conversion factor $X_{\rm CO}$
using absorption of diffuse X-ray emission by local molecular clouds was
developed. It was applied to the Ophiucus (G353+17) and Corona Australis
(G359-18) clouds using CO-line and soft X-ray archival data. We obtained a
value $X_{\rm CO} =1.85 \pm 0.45 \times 10^{20} {\rm H_2 cm^{-2}/(K~ km~
s^{-1})}$ as the average of least-$chi^2$ fitting results for R4 (0.7 keV) and
R5 (0.8 keV) bands.
  Full resolution pdf available at
http://www.ioa.s.u-tokyo.ac.jp/~sofue/htdocs/2016Xco/
