A secondary battery built in a bio-signal measuring instrument (a wearable biosensor) is charged without using a dedicated charging terminal. A bio-signal measuring device includes: a battery as an internal power source and a charging circuit for the battery; electrodes which are brought into contact with the skin surface of the 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 a bio-signal and feed power distribution device, in which the bio-signal processing circuit and the charging circuit are switchably connected to the electrodes through the bio-signal and feed power distribution device, and at the time of the charging of the battery, the electrodes are used as charging terminals.