To provide a flow passage structure having a reserve tank that is lightweight and has simple constitution and also has a superior air trapping function, and a liquid-cooling system using the same.

A liquid circulation flow passage 20 is formed between a pair of opposite flow passage plates 20U and 20L, an outflow nozzle 40 which communicates with the liquid circulation flow passage 20 and protrudes downward is formed integrally with the flow passage plate 20L disposed below in an in-use state between the pair of flow passage plates 20L and 20U, and a liquid tank 30 enclosing the outflow nozzle 40 is provided below the flow passage plate 20L, which has a liquid inflow hole 50 linking a liquid tank 30 and the liquid circulation passage 20 to each other and linked to the outflow nozzle 40 through the liquid tank 30.