The roller brake mounting adapter is configured to be mounted to a bicycle wheel driving device (e.g., hub) equipped with an internal motor. The roller brake mounting adapter has a flange part and a cylindrical part. The flange part is configured to be mounted to a side end section of a hub shell of the hub. The cylindrical part has a first contact surface and a second contact surface. The first contact surface is provided on an internal circumferential surface of the cylindrical part, and configured to contact a bearing serving to rotatably support the hub shell on a shaft of the hub. The second contact surface is provided on an external circumferential surface of the cylindrical part, and has protrusions that are spaced apart in a circumferential direction. The second contact surface is configured to contact a brake drum of the roller brake device.