  The rotation curve of the Galaxy is generally thought to be flat. However,
using radial velocities from interstellar molecular clouds, which is common in
rotation curve determination, seems to be incorrect and may lead to incorrectly
inferring that the rotation curve is flat indeed. Tests basing on photometric
and spectral observations of bright stars may be misleading. The rotation
tracers (OB stars) are affected by motions around local gravity centers and
pulsation effects seen in such early type objects. To get rid of the latter a
lot of observing work must be involved. We introduce a method of studying the
kinematics of the thin disc of our Galaxy outside the solar orbit in a way that
avoids these problems. We propose a test based on observations of interstellar
CaII H and K lines that determines both radial velocities and distances. We
implemented the test using stellar spectra of thin disc stars at galactic
longitudes of 135{\degr} and 180{\degr}. Using this method, we constructed the
rotation curve of the thin disc of the Galaxy. The test leads to the obvious
conclusion that the rotation curve of the thin gaseous galactic disk,
represented by the CaII lines, is Keplerian outside the solar orbit rather than
flat.
