A method for performing notification for resource subscription by a machine-to-machine, M2M, device, the method comprising: detecting a change in a subscription target source comprising a subscription resource as a child resource; generating a notification message comprising a value indicating an event category of the change according to second attribute information configured in the subscription resource; and determining (S1706, S1714) reachability to a receiving device based on scheduling information configured in a scheduling resource for the M2M device and scheduling information configured in a scheduling resource for the receiving device, wherein: when the second attribute information is set to 'immediate' and it is determined that the receiving device is not reachable according to the scheduling information for the M2M device indicating non-reachable and the scheduling information for the receiving device indicating reachable, the M2M device is converted to a reachable mode and then the notification message is transmitted to the receiving device; when the second attribute information is set to a value other than 'immediate' and it is determined that the receiving device is not reachable according to the scheduling information for the M2M device indicating non-reachable or the scheduling information for the receiving device indicating non-reachable, the notification message is processed by the M2M device according to a value of first attribute information configured in the subscription resource and the processed notification message is transmitted to the receiving device after the receiving device is recovered into a reachable mode (S1710, S1718), wherein the processing of the notification message comprises storing a most recently generated notification message and discarding a remaining notification message when the first attribute information is set to sendLatest. detecting a change in a subscription target source comprising a subscription resource as a child resource; generating a notification message comprising a value indicating an event category of the change according to second attribute information configured in the subscription resource; and determining (S1706, S1714) reachability to a receiving device based on scheduling information configured in a scheduling resource for the M2M device and scheduling information configured in a scheduling resource for the receiving device, wherein: when the second attribute information is set to 'immediate' and it is determined that the receiving device is not reachable according to the scheduling information for the M2M device indicating non-reachable and the scheduling information for the receiving device indicating reachable, the M2M device is converted to a reachable mode and then the notification message is transmitted to the receiving device; when the second attribute information is set to a value other than 'immediate' and it is determined that the receiving device is not reachable according to the scheduling information for the M2M device indicating non-reachable or the scheduling information for the receiving device indicating non-reachable, the notification message is processed by the M2M device according to a value of first attribute information configured in the subscription resource and the processed notification message is transmitted to the receiving device after the receiving device is recovered into a reachable mode (S1710, S1718), wherein the processing of the notification message comprises storing a most recently generated notification message and discarding a remaining notification message when the first attribute information is set to sendLatest. when the second attribute information is set to 'immediate' and it is determined that the receiving device is not reachable according to the scheduling information for the M2M device indicating non-reachable and the scheduling information for the receiving device indicating reachable, the M2M device is converted to a reachable mode and then the notification message is transmitted to the receiving device; when the second attribute information is set to a value other than 'immediate' and it is determined that the receiving device is not reachable according to the scheduling information for the M2M device indicating non-reachable or the scheduling information for the receiving device indicating non-reachable, the notification message is processed by the M2M device according to a value of first attribute information configured in the subscription resource and the processed notification message is transmitted to the receiving device after the receiving device is recovered into a reachable mode (S1710, S1718), wherein the processing of the notification message comprises storing a most recently generated notification message and discarding a remaining notification message when the first attribute information is set to sendLatest.