A gas-sensing element configured to measure a concentration of a specific component of a gas is mounted to a first circuit board which includes a driving circuit configured to drive the gas-sensing element. A moisture-proof material is disposed over at least one side of the first circuit board disposed in a tubular gas-sensing element case fixed to a sensor case. A gas-sensing chamber is defined by the first circuit board and an inner tubular surface of the gas-sensing element case, and opens at an open end of the gas-sensing element case to receive the gas to be monitored. A second circuit board which includes a control circuit configured to control the gas-sensing element via the driving circuit is fixed to a sensor case, and disposed in a position separate from the gas-sensing chamber such that the second circuit board is kept out of contact with the gas to be monitored.