A reproduced signal evaluation method employed in a decoding operation, wherein the decoding operation uses a PRML method that decodes binarized bitstreams generated from reproduced signals obtained from an information-recorded medium, onto which the information is modulated using a run length limited code with shortest run length 2T, wherein the PRML method involves generating (S201-203) a plurality of evaluation bitstreams describing a state to which a current binarized bitstream will change, evaluating (S204-S207) for each of the plurality of evaluation bitstreams a likelihood, with which the current binarized bitstream will change to the evaluation bitstreams, based on Euclidean differences between the evaluation bitstreams and the current binarized bitstreams, determining the binarized bitstream after the state change and repeating the determination procedure to decode the reproduced signals; wherein the evaluation bitstreams are expressed as a combination of a main bitstream and sub bitstreams, the main bitstream being defined to be a portion that does not change according to a restraint length of the PRML, the sub bitstreams being added to both of front and rear end of the main bitstream; wherein, in executing the decoding operation, the reproduced signal evaluation method generates a first evaluation bitstream corresponding to a most likely first state change and a second evaluation bitstream corresponding to a second most likely second state change; in generating the first evaluation bitstream and the second evaluation bitstream, prepares in advance a plurality of bit patterns describing the main bitstream by comprising four particular main bitstreams satisfying the run length limited code condition and of length L=5+2i, where i is an integer greater or equal to 0 and is incremented for each of the plurality of bit patterns and specifies the number of successive 2T's included in the main bitstream; compares the main bitstreams and a part of the current binarized bitstream and selects from among the plurality of bit patterns a first main bitstream applicable to the first evaluation bitstream; executes a predetermined calculation on the main bitstream applicable to the first evaluation bitstream to generate a second main bitstream applicable to the second evaluation bitstream; and attaches the sub bitstreams to the first main bitstream and the second main bitstream to generate the first evaluation bitstream and the second evaluation bitstream. generating (S201-203) a plurality of evaluation bitstreams describing a state to which a current binarized bitstream will change, evaluating (S204-S207) for each of the plurality of evaluation bitstreams a likelihood, with which the current binarized bitstream will change to the evaluation bitstreams, based on Euclidean differences between the evaluation bitstreams and the current binarized bitstreams, determining the binarized bitstream after the state change and repeating the determination procedure to decode the reproduced signals; generates a first evaluation bitstream corresponding to a most likely first state change and a second evaluation bitstream corresponding to a second most likely second state change; in generating the first evaluation bitstream and the second evaluation bitstream, prepares in advance a plurality of bit patterns describing the main bitstream by comprising four particular main bitstreams satisfying the run length limited code condition and of length L=5+2i, where i is an integer greater or equal to 0 and is incremented for each of the plurality of bit patterns and specifies the number of successive 2T's included in the main bitstream; compares the main bitstreams and a part of the current binarized bitstream and selects from among the plurality of bit patterns a first main bitstream applicable to the first evaluation bitstream; executes a predetermined calculation on the main bitstream applicable to the first evaluation bitstream to generate a second main bitstream applicable to the second evaluation bitstream; and attaches the sub bitstreams to the first main bitstream and the second main bitstream to generate the first evaluation bitstream and the second evaluation bitstream.