We propose a novel compression technique on signs of DCT coefficients. The proposed technique utilizes a high correlation among pixels at boundaries of a current block and those of neighboring blocks and estimates the original signs. It encodes the difference between the original coefficient signs and the estimated signs. We also propose a speeding up technique for the estimation, utilizing concepts of Gray code and transform matrix decomposition. Compared to JM16.2, the proposed technique reduces 10% bitrate in sign bits of DCT coefficients and achieves 1% BD-bitrate reduction in total bitstream. With the speeding up technique, complexities increase only 8% and 5% on an encoder and a decoder side, respectively.