A method of sterilizing a preform includes steps of: sterilizing bacteria adhering to a preform (1) made of resin by gasifying a sterilizer, discharging a sterilized gas (G) from a nozzle (6) toward a preform (1), now traveling, so as to adhere to the preform (1)； activating the sterilizer adhering to the preform (1) by blasting hot air (P) to the preform； and sterilizing bacteria adhering to the preform, and removing the sterilizer adhering to the preform (1), wherein the above steps are performed sequentially in order. According to the above-mentioned steps, the sterilizer does not enter a blow-molding machine.
