  We study the nature of domain walls in an ordered phase in the
phase-competing region of two Ising-type order parameters. Considering a
two-component $\phi^4$ theory, we show that the domain wall of the ground-state
(primary) order parameter shows a second-order phase transition associated with
the secondary order parameter of the competing phase; the effective theory of
the phase transition is given by the Landau theory of Ising-type phase
transition. We find that the phase boundary of this phase transition is
different from the spinodal line of the competing order. Experimentally, the
phase transition is detected by the divergence of the susceptibility
corresponding to the secondary order when the temperature is quenched to
introduce the domain walls.
