the photodetector 24 to issue a response signal accordingly on the other hand the light entering the reference pathway 222 will be block from contacting the gases to be sensed since the optic gaps thereof is sealed by the isolating element 2221 sop that the intensity of the light is maintained unchanged and then the intensity unchanged light pass the filter 23 and enter the photodetector 24 for enabling the photodetector 24 to issue a reference signal accordingly the filter 23 is used to isolate lights other than the intended light emitted from the photogenerator 21 from entering the photodetector 24 and the photodetector 24 is used to convert the received response reference signal into a corresponding electric signal in addition in order to prevent the accuracy of the apparatus 2 of the invention to be adversely affected by the pollution of dust or dirt depositing in the optic gaps a separation film is provided for isolating dust and dirt from entering the apparatus 2 while allowing the plural gases to pass through please refer to fig4 which is a schematic illustration of an apparatus for sensing plural gases according to another preferred embodiment of the invention the apparatus for sensing plural gases 3 is formed on a substrate 30 that is basically used for detecting three different gases the structure of the apparatus 3 is similar to the apparatus 2 shown in fig2 a and the only difference between the two is that the apparatus 3 has three planar lightwave circuit and the devices corresponding thereto the apparatus 3 has a first planar lightwave circuit 24 a second planar lightwave circuit 35 and a.