The contents of gangue minerals, such as Al2O3, P2O5, crystalline water and so forth in the iron ore imported to Japan tend to increase year by year. Though the increments of such gangue contents are approximately 0.1-0.2 mass%, some problems for the blast furnace operation in ironmaking process have already been reported. In order to clarify the mechanism of harmful effect caused by the small increment of gangue minerals, mainly Al2O3, on the blast furnace operation, the dripping behavior of FetO-CaO-SiO2-Al2O3-MgO slag from the iron funnel, which simulated the micro-porosity in sinter ore, has been observed with the wide range of basicity, Al2O3, FetO and MgO contents. The effect of MgO on the dripping behavior of slag has been summarized in this paper. It was found that the addition of 2mass% MgO resulted in large increase of hold-up on the most of the conditions. When FetO content was less than 20mass% and the basicity was lower than 0.8, MgO lowered the hold-up even if the slag contained 10mass% Al2O3. Every slag hold-up showed the minimum at the basicity of less than unity on the conditions of constant FetO and Al2O3 contents. This trend became remarkable when the slag contained 2 mass% MgO.