PURPOSE: To eliminate the influences of adjacent magnetic fields produced at a nonmagnetized bit section by providing a magnet for compensation at the nonmagnetized bit section between same-pole bits so that the magnetic fields produced from the magnets of the same-pole bits can be offset.

CONSTITUTION: A magnet code plate 10 is constituted of plural N-pole magnets 121 and 122 and a magnet 13 for compensation provided between the magnets 121 and 122. In other words, the magnets 121 and 122 are provided at the positions of N-pole bits and the magnet 13 is installed to the position of a nonmagnetized bit. The magnet 13 is magnetized in a stripe-like state partially as shown by the broken lines in the figure. The magnetization of the magnet 13 is made to N-pole when both sides of the nonmagnetized bit are N-poles and to S-pole when both sides are S-poles. By offsetting the S(N)-pole magnetic field produced at the position of the nonmagnetized bit between the N(S)-pole bits of the same polarity with the magnetic field from the magnet 13 in such way, erroneous detection can be prevented.