  We examine the interpretation of the light scalar meson nonet as tetraquark
states using QCD sum rules. With the interpolating current for the tetraquark
states composed of scalar diquark and scalar antidiquark, first, we construct
the QCD sum rules by means of the operator product expansion up to the
operators of dimension 8 and show that there is no evidence of the coupling of
the tetraquark states to the light scalar meson nonet. In order to have a
stable sum rule, we propose a "good" interpolating current for the tetraquarks
based on chirality arguments which includes scalar and pseudoscalar
diquark--antidiquarks with equal weights. In particular, for the lowest
tetraquark $\sigma$--meson, we perform detail analysis of the QCD sum rule and
obtain mass for the $\sigma(600)$ around 780 MeV.
