PROBLEM TO BE SOLVED: To provide a steam sterilization automatic on-off valve device for a high pressure fluid circuit capable of changing-over between a high pressure processing and an automatic steam sterilization processing.SOLUTION: There are provided a stepped piston 11 with a large-diameter piston part 12 and a small-diameter piston part 13 a stepped cylinder 16 having a large diameter cylinder part 14 in which the large-diameter piston part 12 of the stepped piston 11 is slidably inserted in it and a small-diameter cylinder part 15 in which the small-diameter piston part 13 is slidably inserted in it and a piston reciprocating means 20 for reciprocating the stepped piston 11 inserted into the stepped cylinder 16. The upper end of the small-diameter cylinder part 15 has a communication hole 31 communicated with a flow passage from a high pressure fluid circuit 30. The small-diameter piston part 13 has a closing piece 21 for closing the communication hole 31 when the stepped piston 11 slides toward the small-diameter piston part 13 and in turn for releasing the communication hole 31 when the stepped piston 11 slides toward the large-diameter piston part 12.
