The present invention relates to a rainwater collecting device using a rainwater discharge pipe installed on a wall of a building which is connected to the end of the rainwater discharge pipe discharging rainwater from the roof of the building. The rainwater collecting device includes a water collecting container which includes a discharge pipe joining hole where the rainwater discharge pipe is fitted and an internal space isolated from the outside; a partitioning plate, which is formed inside the water collecting container and which transversely partitions off the internal space, wherein a storage unit formed on the lower side of the discharge pipe joining hole and a drain unit consisting of the remaining space are partitioned off; and a drain hole which is formed on one side of the water collecting container to be connected to the drain unit. The rainwater collecting device is advantageous for obtaining enough water even with rainfall over a short period of time by collecting rainwater from the rainwater discharge pipe where rainwater in a wide area is gathered and discharged.