  Magnetic susceptibility and specific heat measurements in magnetic fields
were performed on an $S=1/2$ one-dimensional antiferromagnet KCuGaF$_6$.
Exchange interaction was evaluated as $J/k_{\rm B}\simeq 100$ K. However, no
magnetic ordering was observed down to 0.46 K. It was found that an applied
magnetic field induces a staggered magnetic susceptibility obeying the Curie
law and an excitation gap, both of which should be attributed to the
antisymmetric interaction of the Dzyaloshinsky-Moriya type and/or the staggered
$g$-tensor. With increasing magnetic field $H$, the gap increases almost in
proportion to $H^{2/3}$.
