PURPOSE: To reduce pressure without requiring power as well as without discharging air in a space flying body such as a spacecraft outside, and return the air, discharged at the pressure reducing time, into the space flying body at the repressurizing time.

CONSTITUTION: Air in a spacecraft 1 is discharged to a variable displacement volume part 7, additionally provided outside of the spacecraft 1, utilizing pressure difference between the spacecraft 1 and external environment 3 so as to reduce pressure in the spacecraft 1. At the repressurizing time in the spacecraft 1, the discharged to the volume part 7 is returned into the spacecraft 1 by a pump 9.