  We analyze the temporal evolution of the three-dimensional (3D) magnetic
structure of the flaring active region (AR) NOAA 10930 by using the nonlinear
force-free fields extrapolated from the photospheric vector magnetic fields
observed by the Solar Optical Telescope on board {\it Hinode}. This AR
consisted mainly of two types of twisted magnetic field lines: One has a strong
negative (left-handed) twist due to the counterclockwise motion of the positive
sunspot and is rooted in the regions of both polarities in the sunspot at a
considerable distance from the polarity inversion line (PIL). In the flare
phase, dramatic magnetic reconnection occurs in those negatively twisted lines
in which the absolute value of the twist is greater than a half-turn. The other
type consists of both positively and negatively twisted field lines formed
relatively close to the PIL between two sunspots. A strong CaII image began to
brighten in this region of mixed polarity, in which the positively twisted
field lines were found to be injected within one day across the pre-existing
negatively twisted region, along which strong currents were embedded.
Consequently, the central region near the PIL distributed with a mix of
differently twisted field lines and the strong currents may play a prominent
role in flare onset.
