In a hydrogen gas station where a great number of sensors are installed, in case hydrogen leakage occurs, sensors in the vicinity of the leakage location start to transmit warning to an access point all at once. This causes channel capacity saturation and failure to transmit warning to the access point. Nodes and the access point are connected each other by an uplink channel and a downlink channel. The access point detects congested/uncongested status of the uplink channel by using a wireless transmitter module and notifies the nodes of the congested/uncongested status of the channel by means for transmitting onto the downlink channel. Meanwhile, each node controls its transmission operation, according to the hydrogen concentration level detected by its hydrogen sensor and the uplink channel congested/uncongested status notified from the access point. In this way, channel overflow due to access congestion in case of hydrogen leakage can be prevented.