Continuous observations were obtained of NOAA AR 10953 with the Solar Optical Telescope (SOT) on board the Hinode satellite from 2007 April 28 to May 9. A prominence was located over the polarity inversion line (PIL) to the southeast of the main sunspot. These observations provided us with a time series of vector magnetic fields on the photosphere under the prominence. We found four features: (1) The abutting opposite-polarity regions on the two sides along the PIL first grew laterally in size and then narrowed. (2) These abutting regions contained vertically weak but horizontally strong magnetic fields. (3) The orientations of the horizontal magnetic fields along the PIL on the photosphere gradually changed with time from a normal-polarity configuration to an inverse-polarity one. (4) The horizontal magnetic field region was blueshifted. These indicate that helical flux rope was emerging from below the photosphere into the corona along the PIL under the preexisting prominence. We suggest that this supply of a helical magnetic flux to the corona is associated with evolution and maintenance of active region prominences.