Abstract: Provided are a drain water discharging method and a buoyancy type drain trap in which a float is accommodated in a case and drain water allowed to flow into the case can be discharged by buoyancy acting on the float. In a case where an inner diameter of the valve is increased in order to increase a discharge flow rate at a valve connected to an outlet port formed in the case, the movement of a valve seat for opening the valve is facilitated through addition of some force to increase the buoyancy or through a reduction in a weight of the float itself.