It is increasingly important to environments surrounding our daily life to monitor chemicals present in the environments. 
For example, recently environmental pollution caused by traces of chemicals, such as dioxine, etc.,  discharged from refuse incineration facilities is noted. 
Instances that chemicals called VOC (Volatile Organic Compound) contained in construction materials of new houses, mansions evaporate in the rooms to damage the dwellers' health are reported, which is a serious problem called a sick house syndrome. 
It is urgently necessary to monitor with high sensitivity and accuracy the traces of chemicals present in the environments as described above to thereby identify sources of the chemicals, and control discharge amounts of the chemicals to the environments.  To this end, it is urgently necessary to develop environmental sensors which can detect the chemicals in real time with high sensitivity and identify chemicals present in environments. 
As conventional methods for measuring chemicals in atmospheres, heating leaving GC-MS (Gas Chromatography-Mass Spectroscopy) and FTIR (Fourier Transform Infrared Spectroscopy), etc.  are known. 
In the heating leaving GC-MS, first a gas-to-be-monitored is adsorbed on a porous material, such as TENAX or others.  Then, the porous material is heated to leave chemicals adsorbed on the porous material, and components of the chemicals are identified and determined by a mass spectroscope.  Thus, component separation and structural analysis of traces of chemicals can be made in a series of measurements. 
In the FTIR, as shown in FIG.  19, infrared light is applied from an infrared light source 100 to a gas-to-be-monitored.  Then, the infrared light which has passed through the gas-to-be-monitored is spectroscopically analyzed by a spectroscope 102, and absorbance spectra are given.  Infrared absorbance spectra are intrinsic to chemicals, which enables the chemicals in the gas-to-be-monitored to be identified.  Absorbed amounts of the infrared light are proportional to concentration of chemicals, which enables the quantities of chemicals in the gas-to-be-monitored to be determined.  The FTIR is simpler in the device structure than the GC-MS and takes a shorter period of time for the measurement.  The FTIR is a real time measuring method.  Furthermore, advantageously, the FTIR is brought into an environment to be monitored and can monitor the environment at the site. 
However, the GC-MS takes several hours for the usual measurement.  From the viewpoint of environmental monitoring, the GC-MS is not usable in real time.  Columns used in the GC-MS must be prepared in laboratories, and cannot monitor the environment at the site.  Thus, it is difficult to effectively feed the back monitored results to environmental control, etc. 
On the other hand, disadvantageously the FTIR is difficult to monitor atmospheric environments with high sensitivity.  Generally, when an atmospheric environment is monitored by FTIR, as described above, the infrared light source and the spectroscope are installed in the atmospheric environment to be monitored, and infrared light is applied directly to the atmosphere to spectroscopically analyze the infrared light. 
In this case, a monitoring sensitivity of the FTIR to chemical present in a gas is proportional to an optical path length of the applied infrared light.  For example, to detect a chemical present in an about 1 ppm concentration in a gas, an optical path length of the infrared light must be about 1 m. Thus, in order to detect a trace of a chemical which is present in an atmosphere only in a concentration ratio of 0.01 ppm, i.e., with higher sensitivity, even a 100 m-optical path length of the infrared light is required.  In other words, in order to detect traces of chemicals by the FTIR, a large-scale optical system which ensures the optical path length of the infrared light is required.  Thus, in realizing the detection of traces of chemicals in atmospheres by the FTIR, the advantage of the FTIR that the FTIR can be installed at site and can measure in real time is spoiled.