A biological signal processor comprising: a sensor unit that is usable with a mattress, and adapted to measure biological displacement of a subject on the mattress, and to output a measurement signal indicative of a measurement result; a signal processing unit adapted to amplify the measurement signal with a plurality of different gains and outputs respective output signals; an AD converting unit adapted to perform AD conversion on the respective output signals to obtain level values and outputs the respective level values; and a determining unit adapted to determine that the sensor unit is in an abnormal condition in case where a standard deviation, a dispersion, or a variation coefficient of a level value corresponding to an output signal amplified with a minimum gain among the respective level values is equal to or less than a predetermined value, characterized in that the determining unit is adapted to use the level value corresponding to the output signal amplified with the minimum gain among the respective level values as a reference value, and calculate a ratio of a standard deviation, a dispersion, or a variation coefficient of a level value corresponding to an output signal amplified with another gain with the standard deviation, the dispersion, or the variation coefficient of the reference value, and in case where a ratio of a standard deviation, a dispersion, or a variation coefficient of a level value corresponding to an output signal amplified with a predetermined gain to the standard deviation, the dispersion, or the variation coefficient of the reference value is within a predetermined value range with respect to a ratio of the predetermined gain to the minimum gain, the determining unit is configured to employ the output signal amplified with the predetermined gain as the output signal of the signal processing unit. a sensor unit that is usable with a mattress, and adapted to measure biological displacement of a subject on the mattress, and to output a measurement signal indicative of a measurement result; a signal processing unit adapted to amplify the measurement signal with a plurality of different gains and outputs respective output signals; an AD converting unit adapted to perform AD conversion on the respective output signals to obtain level values and outputs the respective level values; and a determining unit adapted to determine that the sensor unit is in an abnormal condition in case where a standard deviation, a dispersion, or a variation coefficient of a level value corresponding to an output signal amplified with a minimum gain among the respective level values is equal to or less than a predetermined value, characterized in that the determining unit is adapted to use the level value corresponding to the output signal amplified with the minimum gain among the respective level values as a reference value, and calculate a ratio of a standard deviation, a dispersion, or a variation coefficient of a level value corresponding to an output signal amplified with another gain with the standard deviation, the dispersion, or the variation coefficient of the reference value, and in case where a ratio of a standard deviation, a dispersion, or a variation coefficient of a level value corresponding to an output signal amplified with a predetermined gain to the standard deviation, the dispersion, or the variation coefficient of the reference value is within a predetermined value range with respect to a ratio of the predetermined gain to the minimum gain, the determining unit is configured to employ the output signal amplified with the predetermined gain as the output signal of the signal processing unit.