A direct pool cooling type passive safety grade decay heat removal method and system for removing core decay heat in a pool type liquid metal reactor when a normal heat removal system breaks down. In the liquid metal reactor comprising a reactor vessel, the interior of which is partitioned into a hot pool above a core and a cold pool around the core so that liquid level difference between the hot pool and the cold pool is maintained by a primary pumping head under normal steady-state conditions, is disposed at least one circular vertical tube in such a manner that the sodium in the circular vertical tube is maintained with the same liquid level as the liquid level of the sodium in the cold pool. In the circular vertical tube is disposed a sodium-sodium heat exchanger, which is connected to a sodium-air heat exchanger mounted above a reactor building via a heat removing sodium loop, in such a manner that it is placed at the position higher than a liquid level of the sodium in the cold pool under the normal steady-state conditions.