To provide a rainwater storage tank making effective use of rainwater as general service water and used as each house storage facility for an outflow restraining measure which is one of integrated flood control measures.

A rainwater storage tank body 1 is composed of an upper part and a lower part. The upper part is provided with 90° bent pipes 3, 4 allowing rainwater to flow in and out on a straight line in a vertical direction, an overflow weir 5 for overflowing when the lower part is full, and a filter layer 6 for carrying out simple purification of rainwater. The lower part is provided with a storage part, a faucet 10 for taking out rainwater as general service water, and a drain cock 11 for emptying the inside for maintenance. A connecting pipeline 9 is provided to take rainwater into the lower part from the upper part.