A fluid flow control of molten steel in continuous casting mold directly leads to surface and internal quality of slab and product. However, few studies have looked at fluid flow in submerged entry nozzle even when it is a source of flow in the mold. In this study, fluid flow in submerged entry nozzle is in-situ observed through transparent immersion nozzle by a fusible alloy model and a steel flow model. Subsequently, argon gas extract from mold meniscus is also measured at commercial caster.In the fusible alloy model experiment, both potential flow and plug flow emerged depending on argon flow rate, metal flow rate and nozzle diameter. Decreasing argon flow rate, increasing metal flow rate and reducing nozzle diameter leads to a rise of meniscus height in nozzle.Molten steel flow was observed through transparent silica glass nozzle. Both potential-flow and plug-flow also emerged depending on argon flow rate.According to measurement of net argon flow rate through mold meniscus, about 20% of argon gas injected from upper sliding plate is brought into nozzle at molten steel system.Taking these experimental results into consideration, it is concluded that in conventional slab caster, submerged entry nozzle must be filled with molten steel and steel flow must be like plug-flow during usual operation.