  The phase structure of two-flavor QCD is explored for thermal systems with
finite baryon- and isospin-chemical potentials, mu_B and mu_{iso}, by using the
Polyakov-loop extended Nambu--Jona-Lasinio (PNJL) model. The PNJL model with
the scalar-type eight-quark interaction can reproduce lattice QCD data at not
only mu_{iso}=mu_B=0 but also mu_{iso}>0 and mu_B=0. In the mu_{iso}-mu_{B}-T
space, where T is temperature, the critical endpoint of the chiral phase
transition in the mu_B-T plane at mu_{iso}=0 moves to the tricritical point of
the pion-superfluidity phase transition in the mu_{iso}-T plane at mu_B=0 as
mu_{iso} increases. The thermodynamics at small T is controlled by
sqrt{sigma^2+pi^2} defined by the chiral and pion condensates, sigma and pi.
