An object of the present invention is to provide an oil-in-water emulsion defoaming agent that is superior in defoaming properties (initial defoaming property and defoaming durability) and stability (emulsion stability). The present invention provides an oil-in-water emulsion defoaming agent including hydrophilic fumed silica (C), wherein an oil phase (A) is emulsified and dispersed in a water phase (B). Preferably, the content of the oil phase (A) is 10 to 50% by weight, the content of the hydrophilic fumed silica (C) is 0.1 to 6% by weight, and the content of the water phase (B) is 44 to 89.9% by weight based on the weight of the oil phase (A), the hydrophilic fumed silica (C), and the water phase (B). The weight ratio (A/C) of the oil phase (A) to the hydrophilic fumed silica (C) is preferably from 2 to 100.