Water model experiments were carried out to understand the mechanism of mold powder entrapment. The main objective is to clarify an effect of the viscosity of mold powder on the entrapment. Water and silicone oils of different viscosities were used as the working fluids. Onset of the entrapment was judged by eye inspection and using a high-speed camera. The critical water flow velocity at the interface was measured with a laser Doppler velocimetry (LDV) to describe the onset condition. The results were compared with a previously proposed analytical equation without containing the viscosity effect to highlight the effect.