  Using an outer rotation curve of the Galaxy, we explore the galactic
constants and the mass of the Galaxy. We show that \Theta_0 of 200 km/s is more
favorable than the IAU standard value of 220 km/s, and also show that if
\Theta_0 is smaller than 207 km/s the rotation curve beyond 2R_0 is declining
in Keplerian fashion. In the case of \Theta_0= 200 km/s and R_0= 7.6 kpc, the
total mass and the extent of the Galaxy inferred from the rotation curve are
2.0+/-0.3x 10^{11} M_\odot and 15 kpc, respectively. These results may
significantly change the previous view of the Galaxy, that its outer region is
dominated by a massive dark halo extending out to several tens of kpc.
