Provided is a structure for operating a watertight door by use of buoyancy. The structure includes: a detention tank for storing rainwater flowing from a street inlet; a buoyant member which is installed in the detention tank; and a sensor installed in the detention tank, in which if the buoyant member raised by the buoyant of the rainwater collected in the detention tank comes into contact with the sensor, the watertight door is automatically operated. The watertight door structure manually operated at localized heavy rain is improved, and the watertight door is automatically operated if the water level reaches the predetermined level.