A bias voltage generating apparatus is placed outside an ultrasound observation apparatus and used together with the ultrasound observation apparatus, the ultrasound observation apparatus incorporating a transmission circuit generating a transmission signal and a reception circuit performing processing on a received signal, for transmitting/receiving ultrasound to/from a subject using a capacitive transducer. The bias voltage generating apparatus includes a power supply circuit for bias voltage which includes a chargeable secondary battery for producing a bias voltage to be applied to the capacitive transducer and produces the bias voltage, a bias voltage superimposing portion which superimposes the bias voltage on the transmission signal outputted to outside the ultrasound observation apparatus, a charging level detecting portion which detects a charging level of the secondary battery, and a variable gain amplifier which increases a gain for the received signal based on the detected charging level.
