  We propose a method to find the QCD critical point at finite density
calculating the canonical partition function ${\cal Z}_{\rm C} (T,N)$ by
Monte-Carlo simulations of lattice QCD, and analyze data obtained by a
simulation with two-flavor p4-improved staggered quarks with pion mass $m_{\pi}
\approx 770 {\rm MeV}$. It is found that the shape of an effective potential
changes gradually as the temperature decreases and a first order phase
transition appears in the low temperature and high density region. This result
strongly suggests the existence of the critical point in the $(T, \mu_q)$ phase
diagram.
