To examine the neural mechanism of co-speech gesture production, we measured brain activity of bilinguals during an animation-narration task using near-infrared spectroscopy. The task of the participants was to watch two stories via an animated cartoon, and then narrate the contents in their first language (Ll) and second language (L2), respectively. The participants showed significantly more gestures in L2 than in L1. The number of gestures lowered at the ending part of the narration in L1, but not in L2. Analyses of concentration changes of oxygenated hemoglobin revealed that activation of the left inferior frontal gyrus (IFG) significantly increased during gesture production, while activation of the left posterior superior temporal sulcus (pSTS) significantly decreased in line with an increase in the left IFG. These brain activation patterns suggest that the left IFG is involved in the gesture production, and the left pSTS is modulated by the speech load. (Contains 2 figures.)