In the present invention, petroleum is separated into hydrocarbon mixtures having different components at an atomizing step and a collecting step. At the atomizing step, the petroleum is ultrasonically vibrated and is discharged and atomized in a state of an atomized fine particle floating in a carrier gas. At this step, the petroleum is separated into a mixed fluid containing the atomized fine particle and the carrier gas and residual petroleum which is not atomized. At the collecting step, the hydrocarbon mixture is separated and collected from the mixed fluid obtained at the collecting step. In the separating method, the petroleum is separated into the residual petroleum and the mixed fluid at the atomizing step, and the mixed fluid is collected at the collecting step so that the petroleum is separated into hydrocarbon mixtures having different components.