PURPOSE: To provide a BOG treating system and a treating method for a liquefied gas excellent in profitability and capable of easily coping with the high pressurization of the supplying pressure of a BOG from a BOG compressor.

CONSTITUTION: Primary compressors 4 and 5 are provided on a BOG piping 2, for extracting a BOG generated in a storage tank 1 for storing a liquefied gas, a secondary compressor 10 is arranged in series on the discharge side of the primary compressors, and pressure detecting means 12a and 12b are provided on the discharge side of the primary compressors. Based on a detected signal by the pressure detecting means, the set pressure of the first pressure controlling means is set to pressure higher than set pressure in a second pressure controlling means by providing first pressure controlling means 14 and 18 for returning the BOG on the discharge side of the primary compressors to the suction side of the primary compressors via a recycle adjusting valve 13, and second pressure controlling means 17 and 19 for returning the BOG on the discharge side of the secondary compressor to the suction side of the secondary compressor via a recycle adjusting valve 16.