A sound sensor, a hydrophone including the sound sensor, and a pressure balancing device for using for the hydrophone are provided. The hydrophone includes: a sensor including a sensing part and a pressure balancing part; a printed circuit board (PCB) that is electrically connected to the sensor; a case that houses the sensor and the PCB and that has an opening at one side thereof; an elastic membrane that covers the opening of the case; and a signal line that is electrically connected to the PCB to be extended to the outside of the case. The pressure balancing part includes a diaphragm of a thin film and a support that supports the diaphragm. A pressure balancing hole is formed at one side of the pressure balancing part, and at the inside of the pressure balancing part, a channel that is connected to the pressure balancing hole is formed, and a compressible gas is filled in the channel.