Disclosed is a time slot allocation method for reducing consumption of energy in a wireless sensor network, including transmitting and receiving, by an upper node, a report request and a response to and from at least one lower node in order to identify whether there is data to be transmitted between the upper node and the at least one lower node, transmitting, by the upper node, information required for data transmission to the at least one lower node based on the report request and response, and transmitting and receiving, by the upper node, data to and from the at least one lower node based on the information required for data transmission.