Even if a mattress or the like suffers long-term deterioration, a sleep state measuring apparatus is provided which can set a suitable amplification factor A of a biosignal. The sleep state measuring apparatus detects the biosignal which changes depending on the sleep state of a person who gets on the mattress filled with water, amplifies the biosignal, and estimates the sleep state based on the biosignal. A static component P of the mattress internal pressure detected by a biosignal sensor is first obtained (S 11). The mattress internal pressure is the pressure of water in the mattress. From the static component P of the mattress internal pressure, a fluctuation part ΔV of the mattress internal pressure depending on the value is specified (S 12). Each value of the fluctuation part ΔV of the mattress internal pressure is obtained beforehand by applying a predetermined load, and changing the static component P of the mattress internal pressure. The above-described amplification factor A is calculated by correcting predetermined standard amplification factor A0 with the specified fluctuation part ΔV (S 13).