A pulse wave measuring apparatus, comprising: a pressure sensor which is adapted to be pressed above an artery of a subject body; pressing means for applying pressure to said pressure sensor; and pulse wave measuring means for measuring pulse wave generated from said artery based on pressure information output from said pressure sensor in a process of changing the level of the pressure applied by said pressing means to said pressure sensor, said pressing means including pressure adjusting means including a gas container of a constant capacity for adjusting said level of the pressure applied to said pressure sensor using pressure of gas in said gas container, and state changing means which is adapted to set said pressure adjusting means to a gas holding state for holding an amount of the gas in said gas container, to change the gas holding state to a gas isolating state for isolating a partial amount of the gas held in said gas container in said gas holding state from the remaining amount of the gas, to change the gas isolating state to a gas discharging state for discharging said partial amount of the gas in said gas container that is isolated in said gas isolating state, and to change the gas discharging state to said gas holding state for holding said remaining amount of the gas in said gas container, wherein said gas container includes an isolation unit which is adapted to keep said partial amount of the isolated gas, a main unit except for said isolation unit, and a gas flow adjustment unit which is adapted to adjust a gas flow passage between said main unit and said isolation unit, wherein said state changing means is adapted to control said gas flow adjustment unit in said gas holding state for establishing said gas flow passage, and to control said gas flow adjustment unit in said gas isolating state and said gas discharging state for shutting off said gas flow passage, wherein said state changing means is adapted to set said pressure adjusting means to a gas supplying state for supplying gas to said gas container, and wherein said pressure adjusting means further includes a supply source for supplying gas to said gas container in said gas supplying state, wherein said gas flow adjustment unit is a three-port valve having a main- unit-related port for input/output of gas from/to said main unit, an isolation-unit-related port for input/output of gas from/to said isolation unit and a supply-related port for input of gas from said supply source, and wherein said state changing means is adapted to control said three-port valve for connecting said main-unit-related port to said isolation-unit-related port in establishing said flow passage, and to control said three-port valve for connecting said main-unit-related port to said supply-related port in shutting off said flow passage and in said gas supplying state. a pressure sensor which is adapted to be pressed above an artery of a subject body; pressing means for applying pressure to said pressure sensor; and pulse wave measuring means for measuring pulse wave generated from said artery based on pressure information output from said pressure sensor in a process of changing the level of the pressure applied by said pressing means to said pressure sensor, said pressing means including pressure adjusting means including a gas container of a constant capacity for adjusting said level of the pressure applied to said pressure sensor using pressure of gas in said gas container, and state changing means which is adapted to set said pressure adjusting means to a gas holding state for holding an amount of the gas in said gas container, to change the gas holding state to a gas isolating state for isolating a partial amount of the gas held in said gas container in said gas holding state from the remaining amount of the gas, to change the gas isolating state to a gas discharging state for discharging said partial amount of the gas in said gas container that is isolated in said gas isolating state, and to change the gas discharging state to said gas holding state for holding said remaining amount of the gas in said gas container, wherein said gas container includes an isolation unit which is adapted to keep said partial amount of the isolated gas, a main unit except for said isolation unit, and a gas flow adjustment unit which is adapted to adjust a gas flow passage between said main unit and said isolation unit, wherein said state changing means is adapted to control said gas flow adjustment unit in said gas holding state for establishing said gas flow passage, and to control said gas flow adjustment unit in said gas isolating state and said gas discharging state for shutting off said gas flow passage, wherein said state changing means is adapted to set said pressure adjusting means to a gas supplying state for supplying gas to said gas container, and wherein said pressure adjusting means further includes a supply source for supplying gas to said gas container in said gas supplying state, wherein said gas flow adjustment unit is a three-port valve having a main- unit-related port for input/output of gas from/to said main unit, an isolation-unit-related port for input/output of gas from/to said isolation unit and a supply-related port for input of gas from said supply source, and wherein said state changing means is adapted to control said three-port valve for connecting said main-unit-related port to said isolation-unit-related port in establishing said flow passage, and to control said three-port valve for connecting said main-unit-related port to said supply-related port in shutting off said flow passage and in said gas supplying state.