A biological signal processor includes a sensor unit 2 that is usable with a mattress, measures biological displacement of a subject on the mattress, and outputs a measurement signal indicative of a measurement result, a signal processing unit 7 that amplifies the measurement signal with a plurality of different gains and outputs respective output signals, an AD converting unit 8 that performs AD conversion on the respective output signals to obtain level values and outputs the respective level values, and a determining unit 90 that determines that the sensor unit 2 is in an abnormal condition in case where a variation measure indicative of variation degree in 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.
