Synthetic jets are produced by periodic ejection and suction of fluid from an orifice induced by movement of a diaphragm inside a cavity, and a synthetic jet actuator is a useful tool for active flow control. The synthetic jet actuators are low operating power, zero-net-mass-flux and very compact devices which have demonstrated their capability in modifying the subsonic flow characteristics for boundary layer flow control. For the synthetic jet, the effect of the orifice depth on the vortex ring behavior has not been completely clarified yet. In this study, two types of the orifice depth h (h=2 mm and 8 mm) were installed to demonstrate the boundary layer control in a two-dimensional diffuser. It was confirmed that the flow separation can be effectively suppressed for h=2 mm.