The effects of lateral acoustic forcing on a jet diffusion flame is investigated in this work. An H2-CO2 mixture is used as the fuel and its mixing ratio is varied. The mixture is ejected upward from a vertical tube of 1.25 mm in inner diameter and acoustic forcing is applied from its side with a sinusoidal sound wave of 7 kHz in frequency and 115 dB in pressure level. An effect of the forcing shows up in shorter flame height, which varies in accordance with the mixing ratio, or the mean molecular weight of the fuel, and the ejection velocity. The intensity of the flame excitation is evaluated by means of the ratio between the heights of the excited flame and the normal one; higher the intensity of the excitation is, lower the ratio is. It is found that the flame height ratio decreases as the ejection velocity increases until the flow reaches the critical Reynolds number, over which the flow becomes turbulent and the effect of the forcing becomes indistinguishable. It is also found that the ratio decreases as the mean molecular weight decreases while the Reynolds number is kept constant. Relative amplitude of the baroclinic torque is also evaluated on the basis of the experimental results and simple assumptions, and its relation to the flame height ratio is examined; the results infer that the baroclinic torque is not the factor directly causing the decrease of the flame height.