  The sign problem of finite-density QCD at the zero temperature becomes very
severe if the quark chemical potential exceeds half of the pion mass. In order
to understand its property, we consider the sign problem of the one-site
fermion model appearing in its path-integral expression by using the
Lefschetz-thimble method. We show that the original integration cycle becomes
decomposed into multiple Lefschetz thimbles at a certain value of the fermion
chemical potential, which would correspond to half of the pion mass of
finite-density QCD. This triggers a fictitious phase transition on each
Lefschetz thimble, and the interference of complex phases among them plays an
important role for the correct description of the system. We also show that the
complex Langevin method does not work in this situation.
