  The possibility of kaon condensation in high-density symmetric nuclear matter
is investigated including both s- and p-wave kaon-baryon interactions within
the relativistic mean-field (RMF) theory. Above a certain density, we have a
collective ${\bar K}_s$ state carrying the same quantum numbers as the
antikaon. The appearance of the ${\bar K}_s$ state is caused by the time
component of the axial-vector interaction between kaons and baryons. It is
shown that the system becomes unstable with respect to condensation of
$K$-${\bar K}_s$ pairs. We consider how the effective baryon masses affect the
kaon self-energy coming from the time component of the axial-vector
interaction. Also, the role of the spatial component of the axial-vector
interaction on the possible existence of the collective kaonic states is
discussed in connection with $\Lambda$-mixing effects in the ground state of
high-density matter. Implications of $K{\bar K}_s$ condensation for high-energy
heavy-ion collisions are briefly mentioned.
