PROBLEM TO BE SOLVED: To easily and quickly sterilize a preform.SOLUTION: A large number of preforms (1) formed in a bottom-closed cylindrical shape are supplied on a conveyor (4) in an unevenly laid state, the preforms (1) are preheated by blowing heated air (H) to the preforms (1) from both sides of the conveyor (4) while the preforms (1) are made to pass through the inside of a first chamber (5) by the conveyor (4), and the preforms (1) are sterilized by blowing superheated steam (S) to the preforms (1) from both sides of the conveyor (4) while the preforms (1) are consequently made to pass through the inside of a second chamber (6) by the conveyor (4).