Fiber metal laminates (FMLs) consisting of FRP composites and thin metal foils are gathering attention as structural materials for aircrafts. However, it is difficult to apply the conventional ultrasonic method to detect the inside damages because of the total reflection at the first interface between the FRP and the metal. Recently, the authors have developed a new delamination detection method based on the mode conversion of Lamb waves. In this research, therefore, this method was applied to detect the inner delamination in the FML. MFC actuators and FBG sensors were bonded to both top and bottom surfaces of the laminate in order to excite or receive only A modes or S modes separately. The laminates with an artificial delamination were manufactured by embedment of Teflon films in the middle of the thickness, and Lamb waves were propagated through the delaminated area. The results indicated that excited A1 mode was converted into S0 mode in the delaminated area and it returned to A1 mode again after passing through the delamination. Since the velocity dispersion is clearly different between the two modes, the delamination length could be estimated quantitatively from the change in the dispersion of received A1 mode.