  The instanton configuration in the SU(2)-gauge system with a Higgs doublet is
constructed by using the new valley method. This method defines the
configuration by an extension of the field equation and allows the exact
conversion of the quasi-zero eigenmode to a collective coordinate. It does not
require ad-hoc constraints used in the current constrained instanton method and
provides a better mathematical formalism than the constrained instanton method.
The resulting instanton, which we call ``valley instanton'', is shown to have
desirable behaviors. The result of the numerical investigation is also
presented.
