PURPOSE: To enable a gas concentration to be calculated from a cooling temperature by cooling a gas to be measured to its saturation steam pressure temperature and by detecting leakage state according to a generated mist.

CONSTITUTION: A surface of a gas to be measured 10 is roughened and the gas is cooled to its saturation steam pressure temperature, thus enabling a mist 20 to be generated. The mist 20 sets a roughened surface 13 of a detection window 14 which is provided near a temperature controlling means 12 to a leakage state. When the gas 10 passes through the window 14 and the mist 20 is adhered onto the inside roughened inner surface, the window becomes transparent and a transmission factor is increased. Also, a light-emitting element 15 and a light-receiving element 16 are placed opposingly outside a ductwork of a breathing pipe 11 which sandwiches the window 14, thus enabling the leakage state due to the mist 20 of the window 14 to be notified. At that time, a temperature sensor 19 measures a temperature when the element 16 generates leakage due to the mist 20. Then, gas concentration is calculated by using a solvent gas, an element which stores a list of the saturation steam pressure, and a CPU.