BOG treatment system and method for high pressure gas fueled vessel

The present invention provides a system and a method for effectively processing naturally vaporized gas inside a fuel gas tank without using a compressor for a ship using low pressure gas fuel. The pressure of the naturally vaporized gas is increased to a preferable level without using a compressor and also processed to be consumed immediately as fuel. According to the present invention, there is no need to purchase a compressor to process the naturally vaporized gas which is continuously generated inside the fuel gas tank, and this reduces the costs and time spent for purchasing and maintaining the compressor. In addition, the naturally vaporized gas can be immediately consumed as fuel with low costs and less time.