had been converted by the nine to eight converting circuits 15a 15b are supplied to a switch 16 in which they are alternately combined at every bit therefore the switch 16 outputs a digital signal which results from reconverting the two sets of divided digital signals this digital signal is supplied to an output terminal 17 specifically the reproducing head 11 reproduces the signal of c 0 c 1 c 2 c 3 c 4 c 5 from the magnetic tape t this reproduced signal is supplied to the class 4 partial response detecting circuit 13 formed of a suitable means such as a viterbi decoder thus the detecting circuit 13 outputs the digital signal of b 0 b 1 b 2 b 3 b 4 b 5 then the switch 14 divides this digital signal to provide digital signals of b 0 b 2 b 4 and b 1 b 3 b 5 these digital signals are supplied to the nine to eight converting circuits 15a 15b which derive the digital signals of a 0 a 2 a 4 a 1 a 3 a 5 these digital signals are supplied to and alternately selected by the switch 16 thus the digital sinal of a 0 a 1 a 2 a 3 a 4 a 5 are supplied to the output terminal 17 in other words the reproduced signal from the reproducing head 11 is detected by the class 4 partial response detecting circuit 13 formed of a suitable means such as a viterbi decoder the detected digital signal is divided by the switch 14 and supplied to the nine to eight converting circuits 15a 15b in which they are converted and supplied to the switch 16 these digital signals supplied to the switch 16 are alternately selected and supplied.