  The light scalar meson sigma(600) is known to appear at low excitation energy
with very large width on top of continuum states. We investigate it in the QCD
sum rule as an example of resonance structures appearing above the
corresponding thresholds. We use all the possible local tetraquark currents by
taking linear combinations of five independent local ones. We ought to consider
the pi-pi continuum contribution in the phenomenological side of the QCD sum
rule in order to obtain a good sum rule signal. We study the stability of the
extracted mass against the Borel mass and the threshold value and find the
sigma(600) mass at 530 MeV +- 40 MeV. In addition we find the extracted mass
has an increasing tendency with the Borel mass, which is interpreted as caused
by the width of the resonance.
