  Usually, decoherence is generated from the coupling with an outer
environment. However, a macroscopic object generically possesses its own
environment in itself, namely the complicated dynamics of internal degrees of
freedom. We address a question: when and how the internal dynamics decohere
interference of the center of mass motion of a macroscopic object. We will also
show that weak localization of a macroscopic object in disordered potentials
can be destroyed by such decoherence.
