  Highly energetic, variable and distant sources such as Active Galactic Nuclei
provide a good opportunity to evaluate effects due to the emission and the
propagation of high energy photons. In this note, a study of possible
energy-dependent time-lags with PKS 2155-304 light curve as measured by
H.E.S.S. in July 2006 is presented. These time-lags could either come from the
emission processes or also sign a Lorentz Symmetry breaking as predicted in
some Quantum Gravity models. A Cross-Correlation function and a Wavelet
Transform were used to measure the time-lags. The 95% Confidence Limit on the
Quantum Gravity energy scale based on the statistical and systematic error
evaluation was found to be 7x10^17 GeV considering a linear correction in the
standard photon dispersion relations and assuming that emission-induced
time-lags are negligible. For now, this limit is the best ever obtained with a
blazar.
