Provided is a fluid flow device having high freedom of choosing means for detecting flow errors. The fluid flow device includes a channel forming body. The channel forming body forms a plurality of fluid channels, a plurality of detection spaces corresponding to the fluid channels, respectively, and a plurality of communication channels providing respective communications between the fluid channels and the detection spaces corresponding thereto, respectively. Each of the detection spaces contains a detection fluid and a detection gas aligned in a longitudinal direction thereof, and an interface is formed therebetween. The detection gas is contained in the detection space so as to allow the position of the interface to be changed with the pressure change of a processing object fluid that flows through the fluid channels.