To provide a reaction dilute oil of an organohalogen compound and a dispersion oil for an alkali metal capable of stably progressing a decomposing reaction in a method of the decomposing treatment of the organohalogen compound by making the organohalogen compound contact with an alkali metal.

A mineral oil or the like is obtained by solvent refining and hydrorefining on distillate of the reduced-pressure distillation of an atmospheric residue of a paraffin-base crude. The reaction dilute oil and the alkali metal dispersion oil in the decomposition treatment of the organohalogen compound are constituted of a mineral oil which shows a kinematic viscosity of 5-15 mm2/s at 40°C and has a polar substance equal to or less than 0.15 percent by mass by a clay-gel analysis defined in ASTM D 2007.