  The evolution of large-scale magnetic fields in disk galaxies is investigated
numerically. The gasdynamical simulations in a disk perturbed by spiral or bar
potential are incorporated into the kinematic calculations of induction
equations to elucidate the effects of non-axisymmetric disk structure on
magnetic fields. The effects of interstellar turbulence are given as the
turbulent diffusion of magnetic fields. The usually adopted dynamo mechanism of
alpha-effect is not considered in our computations, because it is not obvious
about the actual existence of the effect in a galaxy. Our principal concern is
to clear how observationally and theoretically well-established gas flow
affects the magnetic-field structure and evolution, without putting a lot of
artificial parameters in the model. We have found that the density-wave
streaming motion of gas has a significant influence on the distribution of
magnetic fields: the lines of force are well aligned with spiral arms due to
the compressional and additional shearing flow of gas in these regions.
