PURPOSE: To provide a rainwater drain pipe maintaining drain capacity always to the maximum.

CONSTITUTION: A filter 24 for preventing dirt from flowing into a rainwater drain pipe 20 is fitted to the drain hole 22 of the rainwater drain pipe 20. The upper part of the filter 24 is pierced with a vent pipe 32, and the lower end part of the vent pipe 32 is positioned below the drain hole 22. The vent pipe 32 is designed into such length that the upper end part of the vent pipe 32 is communicated with outside air even in the full state of rainwater at a roof floor part 12. Drain and air thereby flow well in turn in the pipe without generating difference between air pressure in the pipe and atmospheric pressure so as to obtain smooth drainage.