By using the QCD sum rule, we analyze the mass of the scalar meson κ, which has I = 1/2 in the scalar meson nonet of mass less than 1 GeV. The correlator for the meson is calculated under the assumption that the meson κ is in the tetraquark state consisting of a scalar diquarkantidiquark and an pseudoscalar diquark-antidiquark. From the QCD sum rule up to operators of
energy dimension 10 with instanton contributions, we show that the mass of the meson κ may be stable around 730 MeV when the two types of diquark-antidiquark have equal weights and phases.

Keywords: Scalar mesons, QCD sum rule, Diquark, Tetraquark, Instanton