Abstract: A fluid apparatus includes a bellows that is configured to be extendable and contractible in an axial direction in order to suck a fluid from a suction flow passage and eject the fluid to an ejection flow passage. The bellows has peak portions and valley portions, which are alternately formed in the axial direction, and annular side surface portions, which are located between the peak portions and the valley portions and connect the peak and valley portions to each other. A ratio B/A of the thickness B of the axial middle part of each of the peak portions in a direction which is perpendicular to the axial direction, to the thickness A of each of the side surface portions in the axial direction is set within a range of 1.3 to 1.8.