PURPOSE: To purity an oil by securely removing noxious substance therefrom, reuse the product and reduce the formation of soot.

CONSTITUTION: An extract of a plant such as wood, bamboo, grass, algae or seaweeds, a fine powder thereof (including a fine powder of a carbonized product thereof) or a liquefied product thereof and an arbitrary metal such as iron, nickel, cobalt, silicon or aluminum in the form of a simple substance, a complex or a composite material are added to an unpurified oil such as crude oil or waste oil or to a purified oil such as a lubricating oil. An unpurified oil such as crude oil or waste oil is mixed with an extract of a plant, a fine powder thereof or a liquefied product thereof and an arbitrary metal such as iron, nickel, cobalt, silicon or aluminum in the form of a simple substance, a complex, a composite material or a compound, and the noxious substances are removed from the obtained mixture by extraction with a solvent such as water, alcohol or hexane.