We propose a data gathering scheme for a wireless sensor network constructed in a building in this paper. Our system consists of gateway nodes that have a power supply and wired connection to a monitoring PC, and sensor nodes that operate on a battery. Sensor information is periodically gathered from all sensor nodes via gateway nodes to a monitoring PC. To conserve energy, the emission of sensor information is synchronized. A node only needs to turn on its radio transceiver before it emits its own data, to receive sensor information from distant nodes. To have a possible best path to a gateway node, a sensor node in our scheme chooses the next-hop node from gateway nodes and sensor nodes, depending on the stability of radio communications. We confirmed that stable data gathering with a gathering rate of more than 80% could be accomplished in a sensor network in unstable environments.