An exhaust gas treatment method in a slag treatment process in which molten slag produced in a steelmaking step is charged into a slag holding furnace, the molten slag is poured from the slag holding furnace into a closed electric furnace, which accommodates a molten iron layer and a molten slag layer formed on the molten iron layer, the molten slag is continuously reduced in the electric furnace, and valuable materials in the molten slag are collected into the molten iron layer, the method comprising: burning a combustible component in exhaust gas by causing the exhaust gas, which is produced in the electric furnace, to flow into the slag holding furnace and supplying oxygen-containing gas into the slag holding furnace; causing the burned exhaust gas to flow from the slag holding furnace to a suction device through an exhaust gas pipe such that internal pressure of the slag holding furnace and the exhaust gas pipe are negative; adjusting an internal pressure of the electric furnace to be negative in a range between a predetermined upper limit value and a predetermined lower limit value by introducing external air into the exhaust gas pipe through an opening portion provided in the middle of the exhaust gas pipe and varying the flow rate of the external air flowing from the opening portion into the exhaust gas pipe complementarily to the flow rate of the exhaust gas flowing to the opening portion through the inside of the exhaust gas pipe from the slag holding furnace; and decreasing an area of the opening portion when the internal pressure of the electric furnace exceeds the upper limit value, increasing the area of the opening portion when the internal pressure of the electric furnace falls below the lower limit value, and maintaining the area of the opening portion when the internal pressure of the electric furnace does not exceed the upper limit value and when the internal pressure of the electric furnace does not fall below the lower limit value by using an opening area changing unit provided in the opening portion. burning a combustible component in exhaust gas by causing the exhaust gas, which is produced in the electric furnace, to flow into the slag holding furnace and supplying oxygen-containing gas into the slag holding furnace; causing the burned exhaust gas to flow from the slag holding furnace to a suction device through an exhaust gas pipe such that internal pressure of the slag holding furnace and the exhaust gas pipe are negative; adjusting an internal pressure of the electric furnace to be negative in a range between a predetermined upper limit value and a predetermined lower limit value by introducing external air into the exhaust gas pipe through an opening portion provided in the middle of the exhaust gas pipe and varying the flow rate of the external air flowing from the opening portion into the exhaust gas pipe complementarily to the flow rate of the exhaust gas flowing to the opening portion through the inside of the exhaust gas pipe from the slag holding furnace; and decreasing an area of the opening portion when the internal pressure of the electric furnace exceeds the upper limit value, increasing the area of the opening portion when the internal pressure of the electric furnace falls below the lower limit value, and maintaining the area of the opening portion when the internal pressure of the electric furnace does not exceed the upper limit value and when the internal pressure of the electric furnace does not fall below the lower limit value by using an opening area changing unit provided in the opening portion.