  We perform a QCD sum rule study of the open-charmed $D_s (2317)$ as a
four-quark state. Using the diquark-antidiquark picture for the four-quark
state, we consider four possible interpolating fields for $D_s (2317)$, namely,
scalar-scalar, pseudoscalar-pseudoscalar, vector-vector, and
axial-vector--axial-vector types. We test all four currents by constructing
four separate sum rules. The sum rule with the scalar-scalar current gives a
stable value for the $D_s$ mass which qualitatively agrees with the
experimental value, and the result is not sensitive to the continuum threshold.
The vector-vector sum rule also gives a stable result with small sensitivity to
the continuum threshold and the extracted mass is somewhat lower than the
scalar-scalar current value. On the other hand, the two sum rules in the
pseudoscalar and axial-vector channels are found to yield the mass highly
sensitive to the continuum threshold, which implies that a four-quark state
with the combination of pseudoscalar-pseudoscalar or axial-vector--axial-vector
type would be disfavored. These results would indicate that $D_s (2317)$ is a
bound state of scalar-diquark and scalar-antidiquark and/or vector-diquark and
vector-antidiquark.
