  Continuous observations of a flare productive active region 10930 were
successfully carried out with the Solar Optical Telescope onboard the Hinode
spacecraft during 2007 December 6 to 19. We focus on the evolution of
photospheric magnetic fields in this active region, and magnetic field
properties at the site of the X3.4 class flare, using a time series of vector
field maps with high spatial resolution. The X3.4 class flare occurred on 2006
December 13 at the apparent collision site between the large, opposite polarity
umbrae. Elongated magnetic structures with alternatingly positive and negative
polarities resulting from flux emergence appeared one day before the flare in
the collision site penumbra. Subsequently, the polarity inversion line at the
collision site became very complicated. The number of bright loops in Ca II H
increased during the formation of these elongated magnetic structures. The
flare ribbons and bright loops evolved along the polarity inversion line and
one footpoint of the bright loop was located in a region having a large
departure of field azimuth angle with respect to its surroundings. The SOT
observations with high spatial resolution and high polarization precision
reveal temporal change in fine structure of magnetic fields at the flare site:
some parts of the complicated polarity inversion line then disappeared, and in
those regions the azimuth angle of photospheric magnetic field changed by about
90 degrees, becoming more spatially uniform within the collision site.
