Fluid device

A fluid device (215) is arranged in a piston apparatus (21) which is applied to an external combustion engine and which includes a piston main body (211) and a pressure-accumulating chamber (212) that is formed inside the piston main body (211). The fluid device (215) serves to introduce a working medium compressed in a working space of the external combustion engine into the pressure-accumulating chamber (212) and is arranged so that the working medium can flow in an introduction direction toward the pressure-accumulating chamber (212) and an opposite direction of the introduction direction. The fluid device (215) has a channel resistance which is larger for the opposite direction than for the introduction direction. A difference between the channel resistance for the introduction direction and the channel resistance for the opposite direction in the fluid device (215) is based on a shape of the fluid device (215). An inlet portion (215a) of the fluid device (215) facing against the introduction direction is formed to have a relatively large curvature, and is formed to have an opening whose dimension is gradually decreasing toward inside so that the working medium is drawn into the channel (214) of the fluid device (215) along a smooth streamline. An inlet portion (215b) of the fluid device (215) facing against the opposite direction is formed so that a curvature thereof is zero or nearly zero, and an edge is formed at the inlet portion (215b) facing against the opposite direction so as to separate the working medium when the working medium in the pressure-accumulating chamber (212) moves in a direction to flow in the opposite direction.