  We investigate the phase transitions of (2+Nf)-flavor QCD, where two light
flavors and Nf massive flavors exist, aiming to understand the phase structure
of (2+1)-flavor QCD. Performing simulations of 2-flavor QCD with improved
staggered and Wilson fermions and using the reweighting method, we calculate
probability distribution functions in the many-flavor QCD. Through the shape of
distribution functions, we determine the critical surface terminating first
order phase transitions in the parameter space of the light quark mass, heavy
quark mass and the chemical potential, and find that the first order region
becomes larger with Nf. We then study the critical surface at finite density
for large Nf and the first order region is found to become wider with the
increasing chemical potential. On the other hand, the light quark mass
dependence of the critical mass of heavy quarks seems weak in the region we
investigated. The result of this weak dependence suggests that the critical
mass of heavy quark remains finite in the chiral limit of 2-flavors and there
exists a second order transition region on the line of the 2-flavor massless
limit above the tri-critical point. Moreover, we extend the study of 2-flavor
QCD at finite density to the case of a complex chemical potential and
investigate the singularities where the partition function vanishes, so-called
Lee-Yang zeros. The plaquette effective potential is computed in the complex
plane. We find that the shape of the effective potential changes from
single-well on the real axis to double-well at large imaginary chemical
potential and the double-well potential causes the singularities.
