  We study the hadron-quark hybrid equation of state (EOS) of compact-star
matter. The Nambu-Jona-Lasinio (NJL) local SU(3) model with vector-type
interaction is used to describe the quark matter phase, while the relativistic
mean field (RMF) theory with scalar-isovector $\delta$-meson effective field
adopted to describe the hadronic matter phase. It is shown that the larger the
vector coupling constant , the lower the threshold density for the appearance
of strange quarks. For a sufficiently small value of the vector coupling
constant, the functions of the mass dependence on the baryonic chemical
potential have regions of ambiguity which leads to a phase transition in
non-strange quark matter with an abrupt change in the baryon number density. We
show that within the framework of the NJL model, the hypothesis on the absolute
stability of strange quark matter is not realized. In order to describe the
phase transition from hadronic matter to quark matter, the Maxwell's
construction is applied. It is shown that the greater the vector coupling, the
greater the stiffness of the EOS for quark matter and the phase transition
pressure. Our results indicate that the infinitesimal core of the quark phase,
formed in the center of the neutron star, is stable.
