  We investigate cosmological consequences of an extended gravity model which
belongs to the same class studied by Accetta and Steinhardt in an extended
inflationary scenario. But we do not worry about inflation in our model;
instead, we focus on a topological object formed during cosmological phase
transitions. Although domain walls appear during first-order phase transitions
such as QCD transition, they decay at the end of the phase transition.
Therefore the "domain wall problem" does not exist in the suitable range of
pameters and, on the contrary, the "fragments" of walls may become seeds of
dark matter. A possible connection to "oscillating universe" model offered by
Morikawa et al. is also discussed.
