  Kinematics of shocks, ejecta knots, and the compact remnant of a supernova
remnant gives an insight into the nature of the progenitor and surrounding
environment. We report on a proper motion measurement of X-ray knots and rims
of the magnetar-hosting supernova remnant RCW 103. Chandra data obtained in
three epochs, 1999, 2010, and 2016 are used. We find a global deceleration of
12 knots and rims both in northern and southern regions within the last $\sim
24$ yrs, even though its age is thought to be larger than 2 kyr. Some of them
even changed their moving directions from outward ($\sim 1,000$ km s$^{-1}$) to
inward ($\sim -2,000$ km s$^{-1}$). Our findings can be explained with a
collision with a high-density medium both in the northern and southern edges of
the remnant, although the remnant may still be expanding in the wind-blown
cavity. The proper motion of the associated magnetar 1E161348$-$5055 is
possibly detected with a velocity of $\approx 500$ km s$^{-1}$.

High Energy Astrophysical Phenomena