  We discuss spectra of gravitational waves which are originated by the
strongly first order phase transition at the electroweak symmetry breaking,
which is required for a successful scenario of electroweak baryogenesis. Such
spectra are numerically evaluated without high temperature expansion in a set
of extended scalar sectors with additional N isospin-singlet fields as a
concrete example of renormalizable theories. We find that the produced
gravitational waves can be significant, so that they are detectable at future
gravitational wave interferometers such as DECIGO and BBO. Furthermore, since
the spectra strongly depend on N and the mass of the singlet fields, our
results indicate that future detailed observation of gravitational waves can be
in general a useful probe of extended scalar sectors with the first order phase
transition.
