A data transmission path establishing method for establishing a data transmission path from each device to a base station independently to thereby form a network with small devices which are required to consume lower amounts of power such as sensors, a radio communication network system and a sensor network system. A base station transmits detection signals to a plurality of radio devices. Each of the radio devices which have received the detection signal transmits detection response signals each including an identifier that uniquely identifies the device and doubling as a response signal and a detection signal for another radio device. Having received the detection response signal from a radio device, the base station recognizes the radio device as a radio device belonging to the station based on the identifier included in the detection response signal. In the case where a downstream radio device receives a detection response signal from an upstream radio device, the downstream radio device belongs to the upstream radio device. Also the downstream radio device transmits a detection response signal to the upstream radio device. When receiving the detection response signal from the downstream radio device, the upstream radio device transmits a link information signal indicating the identifier of the downstream radio device to the base station.