In a method for producing gas turbine fuel through the step of modifying heavy fuel oil with the use of an asphaltene-insoluble solvent, the utilization factor of the heavy fuel oil usable as gas turbine fuel is increased by making asphaltene selectively removable. A solvent having a specific inductive capacity in the range of 1.4 to 2.0 is used as the asphaltene-insoluble solvent. In particular, water controlled in temperature and pressure so as to have a specific inductive capacity in the above range is used as the asphaltene-insoluble solvent. By using such a solvent, an asphaltene component contained in the heavy fuel oil can be selectively removed and power generation can be performed while utilizing 95% or more of the heavy fuel oil.