A biosignal processing apparatus comprising: a communication interface configured to receive a biosignal, wherein the biosignal is one of an electrocardiogram signal, a photoplethysmogram signal, and an electromyogram signal; and a processor configured to: set a target interval of the biosignal; convert the target interval to a frequency domain signal; define as at least one target component a frequency component that is a frequency component of the target interval corresponding to a set value and is an integral multiple of the set value among frequency components of the frequency domain signal; calculate a quality metric corresponding to the target interval based on the at least one target component and at least one non-target component that is a frequency component of the target interval not corresponding to the set value and is not an integral multiple of the set value among the frequency components; and estimate a quality of the biosignal based on the quality metric, wherein the set value is an integer R≥2; a number of the at least one target component is an integer M≥1; and a number of the at least one non-target component is M*(R-1), and wherein the biosignal is constituted by R repetitions of a basic waveform of the biosignal. a communication interface configured to receive a biosignal, wherein the biosignal is one of an electrocardiogram signal, a photoplethysmogram signal, and an electromyogram signal; and a processor configured to: set a target interval of the biosignal; convert the target interval to a frequency domain signal; define as at least one target component a frequency component that is a frequency component of the target interval corresponding to a set value and is an integral multiple of the set value among frequency components of the frequency domain signal; calculate a quality metric corresponding to the target interval based on the at least one target component and at least one non-target component that is a frequency component of the target interval not corresponding to the set value and is not an integral multiple of the set value among the frequency components; and estimate a quality of the biosignal based on the quality metric, wherein the set value is an integer R≥2; a number of the at least one target component is an integer M≥1; and a number of the at least one non-target component is M*(R-1), and wherein the biosignal is constituted by R repetitions of a basic waveform of the biosignal. set a target interval of the biosignal; convert the target interval to a frequency domain signal; define as at least one target component a frequency component that is a frequency component of the target interval corresponding to a set value and is an integral multiple of the set value among frequency components of the frequency domain signal; calculate a quality metric corresponding to the target interval based on the at least one target component and at least one non-target component that is a frequency component of the target interval not corresponding to the set value and is not an integral multiple of the set value among the frequency components; and estimate a quality of the biosignal based on the quality metric, wherein the set value is an integer R≥2; a number of the at least one target component is an integer M≥1; and a number of the at least one non-target component is M*(R-1), and the set value is an integer R≥2; a number of the at least one target component is an integer M≥1; and a number of the at least one non-target component is M*(R-1), and wherein the biosignal is constituted by R repetitions of a basic waveform of the biosignal. <EOS>