A reproduced signal evaluation method for evaluating a quality of reproduced signals, comprising: a binarized bitstream acquisition step to decode the reproduced signals to time-sequentially acquire binarized bitstreams by using a PRML method, the PRML method comparing the reproduced signals from an information-recorded medium and target signals in a plurality of state changes and selecting a most likely state change; and a predetermined combination detection step to detect a predetermined combination of a most likely state change, first state change, and a secondary most likely state change, second state change; wherein the quality of the reproduced signals is evaluated by the difference between Pa and Pb, where Pa is an index representing a probability of the first state change, Pb is an index representing a probability of the second state change and the difference between Pa and Pb is the Euclidean difference between the two; wherein information recorded in the medium is modulated using a run length limited code with shortest run length of 2T, and the predetermined combination detection step uses a main bitstream group of four main bitstreams satisfying the run length limited code condition and having a length L=5+2i where i represents an integer equal to or greater than 0 and specifies the number of successive 2T's included in the main bitstream, the predetermined combination detection step including: a step of comparing the main bitstreams with a part of the binarized bitstream and selecting a first main bitstream from the main bitstream group; a step of generating a second main bitstream by performing an operation on the first main bitstream by using an XOR mask bitstream; a step of determining a state change including the first main bitstream as the first state change; and a step of determining a state change including the second main bitstream as the second state change. a binarized bitstream acquisition step to decode the reproduced signals to time-sequentially acquire binarized bitstreams by using a PRML method, the PRML method comparing the reproduced signals from an information-recorded medium and target signals in a plurality of state changes and selecting a most likely state change; and a predetermined combination detection step to detect a predetermined combination of a most likely state change, first state change, and a secondary most likely state change, second state change; wherein the quality of the reproduced signals is evaluated by the difference between Pa and Pb, where Pa is an index representing a probability of the first state change, Pb is an index representing a probability of the second state change and the difference between Pa and Pb is the Euclidean difference between the two; wherein information recorded in the medium is modulated using a run length limited code with shortest run length of 2T, and the predetermined combination detection step uses a main bitstream group of four main bitstreams satisfying the run length limited code condition and having a length L=5+2i where i represents an integer equal to or greater than 0 and specifies the number of successive 2T's included in the main bitstream, the predetermined combination detection step including: a step of comparing the main bitstreams with a part of the binarized bitstream and selecting a first main bitstream from the main bitstream group; a step of generating a second main bitstream by performing an operation on the first main bitstream by using an XOR mask bitstream; a step of determining a state change including the first main bitstream as the first state change; and a step of determining a state change including the second main bitstream as the second state change. a step of comparing the main bitstreams with a part of the binarized bitstream and selecting a first main bitstream from the main bitstream group; a step of generating a second main bitstream by performing an operation on the first main bitstream by using an XOR mask bitstream; a step of determining a state change including the first main bitstream as the first state change; and a step of determining a state change including the second main bitstream as the second state change.