  We report the late-time evolution of Type IIb Supernova (SN IIb) 2013df. SN
2013df showed a dramatic change in its spectral features at ~1 year after the
explosion. Early on it showed typical characteristics shared by SNe IIb/Ib/Ic
dominated by metal emission lines, while later on it was dominated by broad and
flat-topped Halpha and He I emissions. The late-time spectra are strikingly
similar to SN IIb 1993J, which is the only previous example clearly showing the
same transition. This late-time evolution is fully explained by a change in the
energy input from the $^{56}$Co decay to the interaction between the SN ejecta
and dense circumstellar matter (CSM). The mass loss rate is derived to be ~(5.4
+- 3.2) x 10^{-5} Msun/yr (for the wind velocity of ~20 km/s), similar to SN
1993J but larger than SN IIb 2011dh by an order of magnitude. The striking
similarity between SNe 2013df and 1993J in the (candidate) progenitors and the
CSM environments, and the contrast in these natures to SN 2011dh, infer that
there is a link between the natures of the progenitor and the mass loss: SNe
IIb with a more extended progenitor have experienced a much stronger mass loss
in the final centuries toward the explosion. It might indicate that SNe IIb
from a more extended progenitor are the explosions during a strong binary
interaction phase, while those from a less extended progenitor have a delay
between the strong binary interaction and the explosion.
