A bias gate part supplies a bias current to a transmission pulse generator. The transmission pulse generator receives the supply 6of the bias current, amplifies an input voltage, and supplies an output voltage obtained by the amplification to array transducer elements. In accordance with the timing of transmitting ultrasound waves from the array transducer elements and the level of the output voltage supplied to the array transducer elements, the bias gate part supplies the bias current to the transmission pulse generator while changing the timing of supplying the bias current.