To safely confirm safety in a space such as a tunnel at a low cost.

First, a worker throws a sensor node 2A from the entrance of the tunnel 1. Next, the worker advances in the tunnel 1 with a terminal 4 after confirming that the terminal 4 does not receive abnormality information from the sensor node 2A. Then, the terminal 4 approaches the sensor node 2A by a worker's hand while calculating distance ds between itself and the sensor node 2A by an RSSI received from the sensor node 2A and displaying the distance. Then, when the distance ds becomes equal to or less than reference distance d0, a message with the fact is output. The worker throws the next sensor node 2B further toward the back when the message is recognized. After confirming that the abnormality information is not received from the sensor node 2B, the worker advances in the tunnel 1 with the terminal 4 with the sensor node 2B as a target. The worker repeats the above processes to confirm the safety to entering distance d.

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