18 and envelope detector 14 system controller 16 consists of an mpu micro processing unit and its program and in addition to the functions of asymmetry detector and sense current controller further includes the functions of controlling switching circuit 20 recorded signal processor 22 prml signal processor 18 pr4 n 3 and reproduced signal processor 24 sense current controller consists of one portion of the functions of system controller 16 and by sense current control circuit 17 that changes the sense current value supplied to mr head 12 according to the commands of system controller 16 envelope detector 14 is made up of a low pass filter 141 that removes the high frequency noise from the reproduced signal and shapes the signal and envelope extraction circuit 142 that extracts an envelope from the reproduced signal that has been shaped by low pass filter 141 amplifying circuit 30 for amplification of the recorded signal and recording thin film head 34 for writing information to magnetic recording medium 32 by means of the recorded signal amplified by amplifying circuit 30 are connected to recorded signal processor 22 mr head 12 is connected to amplifying circuit 36 for amplifying the reproduced signal prml signal processor 18 is of typical construction and.