  Flat rotation curves of spiral galaxies are considered as an evidence for
dark matter, but the rotation curve of the Milky Way is difficult to measure.
Various objects were used to track the rotation curve in the outer parts of the
Galaxy, but most studies rely on incomplete kinematical information and
inaccurate distances. Here, we use a sample of 773 Classical Cepheids with
precise distances based on mid-infrared period-luminosity relations coupled
with proper motions and radial velocities from Gaia to construct the accurate
rotation curve of the Milky Way up to the distance of ~20 kpc from the Galactic
center. We use a simple model of Galactic rotation to measure the rotation
speed of the Sun Theta_0 = 233.6 +/- 2.8 km/s, assuming a prior on the distance
to the Galactic center R_0 = 8.122 +/- 0.031 kpc from the Gravity
Collaboration. The rotation curve at Galactocentric distances 4 < R < 20 kpc is
nearly flat with a small gradient of -1.34 +/- 0.21 km/s/kpc. This is the most
accurate Galactic rotation curve at distances R > 12 kpc constructed so far.
