The present invention provides a NOx-concentration measuring apparatus D for quantitatively analyzing the concentration of NOx contained in a sample gas. Upon receiving a dataword to be transmitted, the system uses an encoding function to encode the dataword into a current codeword in a stream of codewords, wherein the current codeword is less than double the size of the dataword. The passage includes a hexagonal section which is ended at a first side of the annular flange, and a square section which is ended at a second side of the flange. 65 m band, and the wavelength of the pump light that is output from the light source 51 is C-band or L-band. The difference signal is also added to the input signal by a second adding circuit, the output from the second adding circuit is fed to a quantizing circuit which outputs a value kM which is subtracted from the input signal by a second subtracting circuit. The catalyst layer of the cathode has two portions with different water-repelling properties, and a portion of the catalyst layer that does not face a channel has a higher water-repelling property than a portion that faces a channel.