The magnetic head of the present invention has a narrow upper magnetic pole and capable of restricting side erasure and record bleeding. Preferably, the system further comprises a request terminal sending the request for approval to the mediation server by the requester; and an accounting part proceeding with transferring the money if the security transaction server sends the request for approval to the accounting part. A method for measuring frequency-resolved SOP of an optical signal includes tuning and polarization-scrambling a local oscillator, mixing the scrambled LO with the optical signal, and resolving the RF signal in frequency and polarization. This value corresponding to the transmitted torque is corrected on the basis of a value corresponding to the difference between a first distortion indicator value which changes in accordance with the waveform distortion in the first alternating signal and a second distortion indicator value which changes in accordance with the waveform distortion in the second alternating signal.