A fluid machinery includes a rotary mechanism, an annular back pressure chamber and an oil passage. The rotary mechanism includes first and second cooperating members, each including an engaging member extending from an end plate, with the cooperating members being arranged to oscillate relative to each in order to change volumes of operation chambers formed between the cooperating members. The annular back pressure chamber is formed on the back surface side of the end plate of the first cooperating member, and communicates with an intermediate operation chamber of the operation chambers to thrust the first cooperating member against the second cooperating member. The intermediate operation chamber is in an intermediate pressure state. The oil passage is arranged to communicate an oil into the back pressure chamber to fill the back pressure chamber with the oil. The rotary mechanism can include a piston and cylinder, or can include a fixed and orbiting scroll.