We observed the TeV blazar 1ES 1218+304 with the X-ray astronomy satellite Suzaku in May 2006 .
At the beginning of the two-day continuous observation , we detected a large flare in which the 5 - 10 keV flux changed by a factor of \sim 2 on a timescale of 5 \times 10 ^ { 4 } s. During the flare , the increase in the hard X-ray flux clearly lagged behind that observed in the soft X-rays , with the maximum lag of 2.3 \times 10 ^ { 4 } s observed between the 0.3 - 1 keV and 5 - 10 keV bands .
Furthermore we discovered that the temporal profile of the flare clearly changes with energy , being more symmetric at higher energies .
From the spectral fitting of multi-wavelength data assuming a one-zone , homogeneous synchrotron self-Compton model , we obtain B \sim 0.047 G , emission region size R = 3.0 \times 10 ^ { 16 } cm for an appropriate beaming with a Doppler factor of \delta = 20 .
This value of B is in good agreement with an independent estimate through the model fit to the observed time lag ascribing the energy-dependent variability to differential acceleration timescale of relativistic electrons provided that the gyro-factor \xi is 10 ^ { 5 } .