1. A method for updating firmware of a plurality of nodes constituting a wireless sensor network, including a server, a gateway and the plurality of nodes, the method comprising: transmitting firmware to be updated from the server through the gateway to an uppermost node; sequentially transmitting the firmware to the plurality of nodes from the uppermost node to a lowermost node using wireless communication, and the respective nodes, which have received the firmware, recording the firmware in their own memory; and recording the firmware, stored in the memory, in program memory, by running boot programs of the respective nodes when the recording of the firmware is completed in all the nodes, and then constituting a new network with peripheral nodes by executing the firmware recorded in the nodes.