To provide a method and a system for treating liquid fuel in which emission of harmful substances to the environment or generation thereof can be reduced significantly at the time of driving various combustion apparatus while exhibiting high combustion efficiency.

At an appropriate stage between a process for drilling or refining crude oil and an end use process for combusting liquid fuel 10 in a combustion apparatus 11, means 300 for applying magnetic line of force is provided while abutting against the outer wall part of an appropriate tubular body 40 for carrying the liquid fuel 10 or in the proximity thereof. The system 100 for treating liquid fuel further comprises means 301 for controlling the current velocity of liquid fuel 10 flowing through the tubular body 40 being applied with a magnetic field.