A biopotential can be measured with high accuracy without the electrodes coming into direct contact with the skin and without being affected by any motion artifact. The present invention comprises a first lead which detects a biopotential containing noise components, a second lead which is electrically isolated from the first lead and detects noise components, and a differential amplifier circuit which is input with a first signal output from the first lead and a second signal output from the second lead, and which amplifies and outputs a difference between the first signal and the second signal, wherein a value of an input impedance on the second signal side of the differential amplifier circuit is set so that the noise components detected from the second lead will have a frequency that is higher than a frequency spectrum of the biopotential.