A flash vessel whose liquid level rises and drops considerably used in a high pressure acid leach step which includes leaching nickel oxide ore slurry under high temperature and pressure conditions by using an autoclave, and reducing the temperature and pressure of the slurry after leach to the ordinary temperature and ordinary pressure by using the flash vessel, comprising: a hydrostatic tower which is provided with at least one maximum liquid level sensor and at least one minimum liquid level sensor and whose lower part is coupled with an arbitrary position on a slurry outlet pipe between the connection position of the slurry outlet pipe and the flash vessel and the position of a slurry outlet valve, and whose upper part is coupled with an arbitrary position on a vapor outlet pipe, wherein the at least one maximum liquid level sensor is provided at a position at the same level as a maximum liquid level not causing inflow of the slurry into the vapor outlet pipe due to considerable rise and drop of the slurry liquid level within the liquid phase space of the flash vessel to detect a rising liquid level within the hydrostatic tower, and wherein the at least one minimum liquid level sensor is provided at a position at the same level as a minimum liquid level not causing inflow of vapor into the slurry outlet pipe due to considerable rise and drop of the slurry liquid level within the liquid phase space to detect a dropping liquid level within the hydrostatic tower; and a slurry outlet valve controlled opening and closing according to the detection output by the maximum liquid level sensor and the minimum liquid level sensor detecting the liquid level of the hydrostatic tower, opened when the maximum liquid level sensor detects a rising liquid level within the hydrostatic tower and closed when the minimum liquid level sensor detects a dropping liquid level within the hydrostatic tower. a hydrostatic tower which is provided with at least one maximum liquid level sensor and at least one minimum liquid level sensor and whose lower part is coupled with an arbitrary position on a slurry outlet pipe between the connection position of the slurry outlet pipe and the flash vessel and the position of a slurry outlet valve, and whose upper part is coupled with an arbitrary position on a vapor outlet pipe, wherein the at least one maximum liquid level sensor is provided at a position at the same level as a maximum liquid level not causing inflow of the slurry into the vapor outlet pipe due to considerable rise and drop of the slurry liquid level within the liquid phase space of the flash vessel to detect a rising liquid level within the hydrostatic tower, and wherein the at least one minimum liquid level sensor is provided at a position at the same level as a minimum liquid level not causing inflow of vapor into the slurry outlet pipe due to considerable rise and drop of the slurry liquid level within the liquid phase space to detect a dropping liquid level within the hydrostatic tower; and a slurry outlet valve controlled opening and closing according to the detection output by the maximum liquid level sensor and the minimum liquid level sensor detecting the liquid level of the hydrostatic tower, opened when the maximum liquid level sensor detects a rising liquid level within the hydrostatic tower and closed when the minimum liquid level sensor detects a dropping liquid level within the hydrostatic tower.