  The s-wave $K^{\pm}N$ scattering amplitude is computed up to one-loop order
corresponding to next-to-next-to-leading order (or N$^2$LO in short) with a
heavy-baryon effective chiral Lagrangian. Constraining the low-energy constants
by on-shell scattering lengths, we obtain contributions of each chiral order up
to N$^2$LO and find that the chiral corrections are ``natural" in the sense of
viable effective field theories. We have also calculated off-shell $s$-wave
$K^-N$ scattering amplitudes relevant to kaonic atoms and $K^-$ condensation in
``nuclear star" matter including the effect of $\Lambda (1405)$. The $K^-p$
amplitude is found to be quite sensitive to the intermediate $\Lambda (1405)$
contribution, while the $K^-n$ amplitude varies smoothly with the C.M. energy.
The crossing-even one-loop corrections are found to play an important role in
determining the higher-order chiral corrections.
