Latency, measured from the user's motion to the display output, causes virtual reality cybersickness and decreases task performance. However, the effect of small delay has not been investigated sufficiently. Therefore the purpose of this study was to reveal the effect of a small latency on the subjects. The subjects were asked to stand on the force plate with one foot so that the length of body sway can be measured with several lags ranging from 1 ms to 66 ms. The experiments showed that 1) the sway increased linearly as the latency got longer 2) the HMD with latency of 1 ms also degraded the sense of balance compared to the naked eye with the same limited field of view (FOV) as in HMD, and 3) the difference between the virtual and real worlds' content had an effect on the result of the experiment. From these results, user's stability can be regarded as the direct index of the quality of VR system for the user.