in u s pat no 4 606 027 issued aug 12 1986 to s otani and in the article by 34 h kobayashi 34 application of probabilitistic decoding to digital magnetic recording system 34 ibm j res develop vol 15 pages 64 74 january 1971 the term 34 branchmetric 34 indicates ambiguity levels of a change in a state for certain input information for a general magnetic recording system branchmetric is expressed by the logarithm of the probability of a change in a state when noise is assumed to have a white gaussian distribution where s i and s j represent states and y k represents input information including noise since branchmetric includes a common term for respective changes in states each branchmetric can be simplified by erasing the common term according to the above described literature by h kobayashi if simplified branchmetric is expressed by normalized branchmetric the normalized branchmetric for differential equalization can be determined by the following expressions that is equ1 where s 0 and s 1 represent the states in which the recorded waveform is l low and h high in fig2 respectively a branchmetric determination circuit 24 in the present embodiment adds correction term a k in consideration of an equalization error and defines branchmetric by the following expressions for a case in which the same state continues the normalized branchmetric is obtained from the following expressions that is equ3 where a 1 u03b4a a and a 2 u03b4a 2 2a correction terms a 1 and a 2 are determined from the amount of equalization error u03b4a shown in fig2 as shown in fig4 the amount of equalization error u03b4a changes with time when the same state continues accordingly an equalization error amount setting circuit 32 may determine the.