  The very short and bright flare of 3C 279 detected with {\it Fermi}-LAT in
2013 December is tested by a model with stochastic electron acceleration by
turbulences. Our time-dependent simulation shows that the very hard spectrum
and asymmetric light curve are successfully reproduced by changing only the
magnetic field from the value in the steady period. The maximum energy of
electrons drastically grows with the decrease of the magnetic field, which
yields a hard photon spectrum as observed. Rapid cooling due to the
inverse-Compton scattering with the external photons reproduces the decaying
feature of the light curve. The inferred energy density of the magnetic field
is much less than the electron and photon energy densities. The low magnetic
field and short variability timescale are unfavorable for the jet acceleration
model from the gradual Poynting flux dissipation.
