The radial pattern of an ion sound wave in a mercury discharge tube where the ion sound wave is parametrically excited in Tonks‐Dattner resonance is observed. The frequency of the HF (high‐frequency) wave is chosen to be about one order lower than that in Stern's experiment so that the diameter of the tube can be large, by which means the detailed radial pattern of the wave can be obtained. The result qualitatively agrees with the theory predicted by Rosa and Allen‐that is, it does not show the standing‐wave pattern.