To provide a rainwater drainage system which can be constructed using an existing pipeline while reducing labor and time required for construction.

The rainwater drainage system 1 includes: a pit 20 formed with an inflow port 21, an outflow port 22 and an overflow port 23; an inflow pipe 11 connected to the inflow port 21 and leading rainwater toward the pit 20; an outflow pipe 12 connected to the outflow port 22 and allowing rainwater to flow out toward a rainwater main from the pit 20; an overflow pipeline 30 connected to the overflow port 23 and leading rainwater overflowing outside the pit 20 through the overflow port 23; and a rainwater infiltration tank 50 connected to the downstream end of the overflow pipeline 30. The position of the lower end 21b of the inflow port 21 is higher than the position of the lower end 23b of the overflow port 23, and the position of the lower end 23b of the overflow port 23 is higher than the position of the lower end 22b of the outflow port 22.