A bio-signal measuring device comprising: a bio-signal measuring instrument which is used in a state of being mounted to a living body, wherein the bio-signal measuring instrument includes a battery as an internal power source and a charging circuit for the battery, a plurality of electrodes which are brought into contact with a skin surface of a human body at the time of bio-signal measurement and are connected to a predetermined power feeder at the time of charging of the battery, a bio-signal processing circuit which processes bio-signals detected at the electrodes in a predetermined manner, and bio-signal and feed power distribution means, the bio-signal processing circuit and the charging circuit are switchably connected to the electrodes through the bio-signal and feed power distribution means, at the time of the bio-signal measurement, the bio-signals detected at the electrodes are supplied to the bio-signal processing circuit through the bio-signal and feed power distribution means, and at the time of the charging of the battery, feed power which is supplied from the power feeder is supplied to the charging circuit through the bio-signal and feed power distribution means; and characterized by: a first substrate in which the bio-signal processing circuit is mounted on one surface; and a second substrate in which the battery and the charging circuit for the battery are mounted on one surface, wherein an electrode of the plurality of electrodes is provided on the other surface side of each of the first substrate and the second substrate, and the first substrate and the second substrate are connected through a low-bending rigidity part having conductive wiring. a bio-signal measuring instrument which is used in a state of being mounted to a living body, wherein the bio-signal measuring instrument includes a battery as an internal power source and a charging circuit for the battery, a plurality of electrodes which are brought into contact with a skin surface of a human body at the time of bio-signal measurement and are connected to a predetermined power feeder at the time of charging of the battery, a bio-signal processing circuit which processes bio-signals detected at the electrodes in a predetermined manner, and bio-signal and feed power distribution means, the bio-signal processing circuit and the charging circuit are switchably connected to the electrodes through the bio-signal and feed power distribution means, at the time of the bio-signal measurement, the bio-signals detected at the electrodes are supplied to the bio-signal processing circuit through the bio-signal and feed power distribution means, and at the time of the charging of the battery, feed power which is supplied from the power feeder is supplied to the charging circuit through the bio-signal and feed power distribution means; and characterized by: a first substrate in which the bio-signal processing circuit is mounted on one surface; and a second substrate in which the battery and the charging circuit for the battery are mounted on one surface, wherein an electrode of the plurality of electrodes is provided on the other surface side of each of the first substrate and the second substrate, and the first substrate and the second substrate are connected through a low-bending rigidity part having conductive wiring. a first substrate in which the bio-signal processing circuit is mounted on one surface; and a second substrate in which the battery and the charging circuit for the battery are mounted on one surface, wherein an electrode of the plurality of electrodes is provided on the other surface side of each of the first substrate and the second substrate, and the first substrate and the second substrate are connected through a low-bending rigidity part having conductive wiring.