  We perform two-dimensional, magnetohydrodynamical core-collapse simulations
of massive stars accompanying the QCD phase transition. We study how the phase
transition affects the gravitational waveforms near the epoch of core-bounce.
As for initial models, we change the strength of rotation and magnetic fields.
Particularly, the degree of differential rotation in the iron core (Fe-core) is
changed parametrically. As for the microphysics, we adopt a phenomenological
equation of state above the nuclear density, including two parameters to change
the hardness before the transition. We assume the first order phase transition,
where the conversion of bulk nuclear matter to a chirally symmetric quark-gluon
phase is described by the MIT bag model. Based on these computations, we find
that the phase transition can make the maximum amplitudes larger up to $\sim$
10 percents than the ones without the phase transition. On the other hand, the
maximum amplitudes become smaller up to $\sim$ 10 percents owing to the phase
transition, when the degree of the differential rotation becomes larger. We
find that even extremely strong magnetic fields $\sim 10^{17}$ G in the
protoneutron star do not affect these results.
