1. A method of processing a notification message for applying a group policy to a notification subscription in a wireless communication system, the method being performed by a first machine-to-machine (M2M) device and comprising:receiving a plurality of notification messages which are triggered to be generated and transmitted when an event notification criteria of a subscription or a group subscription is satisfied;
determining whether a group subscription ID matching a group subscription ID pre-stored in the first M2M device is included in the received notification messages,
wherein the pre-stored group subscription ID is associated with a group policy applied to the group subscription, and the group subscription is requested and configured by a second M2M device for receiving notification messages for member resources belonging to a specific group; and
aggregating notification messages including the group subscription ID matching the pre-stored group subscription ID using the group policy corresponding to the pre-stored group subscription ID,
wherein the notification messages including the group subscription ID matching the pre-stored group subscription ID are received from group member M2M devices hosting the member resources belonging to the specific group.