The purpose of this study was finding the pain inhibitory effect of acupuncture at $LI_4$. The pain at dentes incisivi was evoked by noxious electric stimulation and EMG changes based on the interval were measured. To do this, the opioid antagonist was administered intraperitoneally. Following are some of results. 1. The pain inhibitory effect of acupuncture at $LI_4$ was 94.35%, 84.56%, 57.62%, 54.40%, at 10, 20, 30 and 40 minutes respectively. All the data were calculated based on 100% at 0 minute. The effect was very significant at 40 minutes. 2. The continuous effect of acupuncture at $LI_4$ after taking out the needle was 58.58%, 57.62%, 66.22%, 73.18%, 83.70%, 92.68%, at 50, 50, 70, 80, 90 and 100 minutes respectively. The overall continuous effect was shown its maximum 20 minutes after taking out the needle and reached to the initial value at 60 minutes. 3. The pain inhibitory effect following the naloxone administration and acupuncture application at $LI_4$ was 95.96%, 96.04%, 94.86%, 94.92% at 10, 20, 30 and 40 minutes respectively. All values showed similar tendency to the initial data. 4. The continuous effect of acupuncture at $LI_4$ after taking out the needle which was preceded by the naloxone administration was 94.48%, 96.02%, 96.02%, 98.00%, 98.46%, 97.18% at 50, 60, 70, 80, 90 and 100 minutes respectively. This trend was not a significant fluctuation. It was concluded that effect of acupuncture at $LI_4$ was shown in conjunction with secretion of analgesic substance. Therefore it is implied that the acupuncture application will play a major role in treating many diseases with more revelation of scientific acupuncture mechanism. Further studies of acupuncture manipulation are needed in the future based on our study.