  We investigate the evolution of two bars formed in fully self-consistent
hydrodynamic simulations of the formation of Milky Way-mass galaxies. One
galaxy shows higher central mass concentration and has a longer and stronger
bar than the other at $z = 0$. The stronger bar evolves by transferring its
angular momentum mainly to the dark halo. Consequently the rotation speed of
the bar decreases with time, while the amplitude of the bar increases with
time. These features qualitatively agree with the results obtained by idealized
simulations. The pattern speed of the stronger bar largely goes up and down
within a half revolution in its early evolutionary stage. These oscillations
occur when the bar is misaligned with the $m = 4$ mode Fourier component. These
oscillations correlate with the oscillations in the triaxilality of the dark
matter halo, but differently from the way identified by idealized simulations.
The amplitude of the weaker bar does not increase despite the fact that its
rotation slows down with time.This result contradicts what is expected from
idealized simulations and is caused by the decline of the central density
associated with the mass loss and feedback from the stellar populations. The
amplitude of the weaker bar is further weakens by the angular momentum
injection by the interactions with stellar clumps in the disk. In the both
galaxies, the bars are terminated around the 4:1 resonance.
