A method of purifying propylene oxide, which comprises subjecting the propylene oxide containing methyl formate as an impurity to extractive distillation using a hydrocarbon of 7 to 10 carbon atoms as an extractant with an extractive distillation column, adding water to the distillate from the overhead of the extractive distillation column to conduct oil-water separation, recycling the oil layer separated to the extractive distillation column, removing the aqueous layer outside the system thereby to obtain propylene oxide in which a methyl formate concentration is reduced as a bottom liquid of the extractive distillation column.