hereinafter , the present invention will be explained in detail with reference to embodiments and accompanied drawings . fig9 shows an example of signal waveforms in a first embodiment of the present invention . in this embodiment , when recording marks on track of a recording medium , the recorded marks m are written in positions shifted by half of a clock in the order of the right neighboring track , the current track and the left neighboring track . in addition , the polarity of the magnetic field of the recorded mark m is set positive (+), while the polarity of the magnetic field of the space s is set negative (−). thus , each of the leading edge and the trailing edge of the recorded mark m is aligned with center portions of the recorded mark m or the space s of the neighboring tracks , and the neighboring tracks have different polarities . in addition , the polarity of the magnetic field applied for reading at the start of writing ( at the leading edge of the recorded mark ) is set positive (+). the effective magnetic field for reading at the leading edge is the sum of the magnetic field applied externally and the leakage magnetic fields of the neighboring tracks . the leakage magnetic fields of the neighboring tracks can be cancelled since they have the opposite polarities . as a result , the effective magnetic field for reading is substantially equal to the magnetic field applied externally . thus , according to this embodiment , the recorded signal can be read correctly with being hardly affected by the leakage magnetic field from the neighboring track . in addition , the applied magnetic field for reading is switched at the center portion of the recorded mark , so that the polarity of the applied magnetic field at the trailing edge of the recorded mark is set negative (−). at the trailing edge too , the leakage magnetic fields from the neighboring tracks are cancelled to be zero in the same way as the leading edge , so the signal can be read with being hardly affected by the leakage magnetic field from the neighboring track . furthermore , by switching the applied magnetic field for reading at the center portion of the recorded mark , the waveform reproduced by the magnetic super resolution ( msr ) method becomes sharp in both leading and trailing edges so that the jitter is reduced . therefore , any of the edges can be used for reproducing the signal . though the applied magnetic field for reading is switched in synchronization with the recorded mark of the neighboring track in fig9 the synchronization is not always required . it is enough that the magnetic field is switched at the timing of detecting the leading edge . fig1 shows an example of signal waveforms in a second embodiment of the present invention . this embodiment is different from the first embodiment shown in fig9 in that the optical head above the current track is shifted ( i . e ., is detracked ) in the direction toward one of the neighboring tracks . fig1 shows an example in which the optical head is detracked to the left neighboring track . as shown in fig1 , being detracked to the left , the influence of the leakage magnetic field from the left track is larger than that from the right track . therefore , leakage magnetic fields of the neighboring tracks are not cancelled and remain the sum of the leakage magnetic fields having the same polarity as the leakage magnetic field from the left track . as a result , the leakage magnetic field strengthens the applied magnetic field for reading . therefore , the applied magnetic field for reading can be set smaller by the value of the leakage magnetic field . in this embodiment too , the applied magnetic field for reading is switched at the center portion of the recorded mark in the same way as the first embodiment , so any of the edges can be used for reproducing the signal . furthermore , the recorded marks are written in positions shifted by half of a clock in the order of the right neighboring track , the current track and the left neighboring track . therefore , the position of the leading edge or the trailing edge of the recorded mark of the current track corresponds to the center portion of the mark or the space of the neighboring track . therefore , even if the optical head is detracked , the above - mentioned problem of cross talk does not occur . fig1 shows an example of signal waveforms in a third embodiment of the present invention . this embodiment realizes the multilevel recording by shifting the position of the leading edge or the trailing edge of the recorded mark in the first or the second embodiment . the clock signal ( clk ) is synchronized with the recorded mark , so that a level detection is performed at the leading edge of the clock signal . in the example shown in fig1 , each position of the leading edge and the trailing edge is changed in three ways , so that three levels can be recorded and reproduced by each edge . however , the number of the level ( the edge positions ) is not limited to three , but can be two levels or more than four levels . alternatively , only one of the leading edge and the trailing edge can be used for the multilevel recording . fig1 shows an example of waveforms in the method of determining the optimal value of the applied magnetic field for reading ( hereinafter , referred to as the optimal reading magnetic field ) according to the present invention . as shown in fig1 , a reference area for determining the optimal reading magnetic field for reading is provided in the recorded data of the recording medium . in the reference area a predetermined pattern of marks is written . the applied magnetic field for reading is increased from the minimum value within a predetermined range when reading the reference area , or is decreased from the maximum value . if the applied magnetic field for reading is increased as shown in fig1 , the amplitude of the read signal increases along with the increase of the applied magnetic field for reading . however , when the applied magnetic field for reading exceeds a predetermined vale , the amplitude of the read signal decreases . therefore , the optimal reading magnetic field is determined to be the applied magnetic field for reading when the read signal is the maximum or to be the intermediate value of the applied magnetic fields for reading when the amplitude of the read signal is above the threshold . the concrete method for the latter is shown in fig1 . fig1 shows an example of waveforms in the method of determining the optimal reading magnetic field to be the intermediate value of the applied magnetic fields for reading when the amplitude of the read signal is above the threshold . as shown in fig1 , the levels s 1 , s 0 and s + 1 at three continuous time points ( the discrete time points ) t − 1 , t 0 and t + 1 are sampled in accordance with the clock signal . using these sampled values s − 1 , s 0 and s + 1 , the intermediate value between the maximum magnetic field and the minimum magnetic field within the range that satisfies the following inequality is determined to be the optimal reading magnetic field . ( s + 1 - s 0 ) 2 +( s − 1 - s 0 ) 2 ≧ x here , x is the threshold value . since the sum of square values is used for determination , fluctuations in the clock can be compensated . fig1 is a block diagram of an example of a magneto - optical recording apparatus utilizing the recording medium and the method of the present invention . a magnetic coil drive circuit 14 drives a magnetic coil 13 so as to apply a magnetic field to a recording medium 11 . the recorded data of the recording medium 11 are read by an optical head 12 so as to obtain a read signal . the read signal is amplified by an amplifier 15 . the gain of the read signal is adjusted by an automatic gain control ( agc ) amplifier 16 , and the waveform of the signal is equalized by an equalizer 17 . in addition , a low pass filter ( lpf ) 18 removes high frequency noises from the signal , which is given to a level detector 25 . the read signal of prepits of the recording medium 11 is given to a phase - locked loop ( pll ) circuit 21 , which generates a clock ( clk ) signal and an inverted clock (* clk ) signal . the clk signal and the * clk signal ( the prefix “*” means the inverted logic ) is given to a selector 23 as an external clock . fig1 shows waveforms indicating the relationship among the recorded mark , the magnetic field for reading and the clock signal in the magneto - optical recording apparatus shown in fig1 . the selector 23 outputs the clk signal and the * clk signal to the a line or the b line in accordance with address information given externally . the a line is connected to a reading magnetic field modulating circuit 24 , while the b line is connected to a level detector 25 . namely , the selector 23 decided whether the current track is an even track or an odd track in accordance with the address information , and outputs the * clk signal to the a line and the clk signal to the b line if it is an even track , while outputs the clk signal to the a line and the * clk signal to the b line if it is an odd track . the address information is also given to the reading magnetic field modulating circuit 24 . the reading magnetic field modulating circuit 24 decides which number the current track is , 4n , 4n + 1 , 4n + 2 or 4n + 3 ( n is zero or a natural number ). the leading edge of the clk signal or the * clk signal that is inputted from the a line is used for switching the polarity of the magnetic field for reading . namely , the polarities of the magnetic field in the tracks 4n , 4n + 1 , 4n + 2 and 4n + 3 are changed as shown in fig1 . furthermore , as explained above , the optimal reading magnetic field is determined in accordance with the read signal of the reference area . thus , a control signal is given to the magnetic coil drive circuit 14 from the reading magnetic field modulating circuit 24 so that the relationship between the edge of the recorded mark and the magnetic field for reading shown in fig1 can be obtained . the level detector 25 detects the level of the read signal at the leading edge of the clk signal or the * clk signal that is input from the b line as shown in fig1 . the detected level is sent to the demodulator 22 and is demodulated . the example shown in fig1 can obtain nine (= 3 2 ) values for one recorded mark since three levels can be detected at leading and trailing edges of the recorded mark . therefore , three - bit information can be demodulated . generally speaking , if the number of the multilevel of the recorded mark is n , n 2 values of information can be obtained from one recorded mark , so that the number of the information bits that can be demodulated corresponds to the maximum value satisfying the inequality , n 2 ≧ 2 m . if n is four , two bit information can be demodulated at each of the leading edge and the trailing edge , so that four - bit information can be demodulated from one recorded mark . alternatively , only one of the leading edge and the trailing edge of the recorded mark can be used for the multilevel demodulating . in the embodiments mentioned above , the magneto - optical recording medium is used . the magnetic recording medium may also be used in the present invention . as explained above , according to the magneto - optical recording medium and the recording and reproducing method of the present invention , the influences of the leakage magnetic field from the tracks neighboring the current track can be cancelled , so that the recorded signal of the current track can be read correctly with being hardly affected by the leakage magnetic field from the neighboring tracks . in addition , according to another method of the present invention , the leakage magnetic field from the neighboring track is used effectively for strengthening the applied magnetic field for reading , which can be set weaker than the optimal reading magnetic field . thus , the energy to generate the magnetic field for reading can be saved . in addition , the magneto - optical recording apparatus according to the present invention can automatically determine the optimal value or the optimal range of the magnetic field for reading , so that the recorded signal can be read correctly .