A sign determination circuit is input with in-phase signal I, and outputs in-phase sign determination signal SPI to indicate the sign, positive or negative, of in-phase signal I. Another sign determination circuit is input with orthogonal signal Q, and outputs orthogonal sign determination signal SPQ to indicate the sign, positive or negative, of orthogonal signal Q. A decoder is input with in-phase sign determination signal SPI and orthogonal sign determination signal SPQ, judging which quadrant the vector represented by in-phase signal I and orthogonal signal Q belongs to, thereby determining I&thgr;4, I&thgr;3 to indicate the 180° bit and 90°-bit, respectively, of angle information I&thgr;. Also, I&thgr;2, I&thgr;1 to indicate the 45°-bit and 22.5°-bit, respectively, of angle information I&thgr; is given by the other decoders.