To provide a method and a system of desulfurization in which sulfur and iron sulfide are easily removed from petroleum such as crude oil or heavy oil and generation of poisonous hydrogen sulfide can be suppressed.

The crude oil or heavy oil is heated and then applied with a high-frequency voltage to allow it to produce crystals of the sulfur and iron sulfide in the crude oil or heavy oil. The crude oil or heavy oil containing the crystals of sulfur and iron sulfide is filtered to remove the crystals of sulfur and iron sulfide by passing the crude oil or heavy oil through a ring tube filter with a construction in which a plurality of flat plate rings are stacked.