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.
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