Cleaning method of oil tank and cleaning device of same

A device of cleaning an inside of an oil tank provided with a roof having an existing opening, the device comprising:

• a nozzle that injects a cleaning liquid comprising LCO (Light Cycle Oil) obtained by distillation of hydrocarbon oil by fluid catalytic cracking to sludge accumulated in the inside of the oil tank;
• a cleaning liquid injection line;
• a circulation line through which the cleaning liquid in which the sludge is dissolved is discharged from the oil tank and returned to the oil tank;
• a discharge line through which the cleaning liquid in which the sludge is dissolved is discharged from the oil tank;
• and
• a cleaning liquid feeding line that is connected to the circulation line and through which the cleaning liquid is fed to the nozzle, the nozzle being capable of being inserted into the existing opening and having an alterable injection posture.