  The resistivity and thermopower of Na$_{1+x}$Co$_2$O$_4$ and
Na$_{1.1-x}$Ca$_x$Co$_2$O$_4$ are measured and analyzed. In
Na$_{1+x}$Co$_2$O$_4$, whereas the resistivity increases with $x$, the
thermopower is nearly independent of $x$. This suggests that the excess Na is
unlikely to supply carriers, and decreases effective conduction paths in the
sample. In Na$_{1.1-x}$Ca$_x$Co$_2$O$_4$, the resistivity and the thermopower
increase with $x$, and the Ca$^{2+}$ substitution for Na$^+$ reduces the
majority carriers in NaCo$_2$O$_4$. This means that they are holes, which is
consistent with the positive sign of the thermopower. Strong correlation in
this compound is evidenced by the peculiar temperature dependence of the
resistivity.
