This study demonstrates an autonomous formation flight of multiple flapping-wing flying vehicles for the first time. In order to extract the position and attitude information of each flying vehicle, an external motion capture system is applied. A path-following controller is designed so that each flapping-wing flying vehicle has a time-independent circular path with the predetermined radius, and the constant forward flight speed and altitude. Due to the variations in flight characteristics among four different flapping-wing flying vehicles, the gain of the directional control, which generates nonlinear rolling moment using the changes of the tension in flexible membrane wings, is individually tuned for each vehicle. The rotational speed of the circular path is kept constant by controlling the desired radius rather than the forward flight speed so that the formation angles between two closest flapping-wing flying vehicles are maintained at 90 degrees. The performances of the individual path-following controllers and the circular formation controller are statistically evaluated in terms of the mean and standard deviation of the flight state variables.