has a protruding portion which fills the gas inlet aperture 44 which communicates with the central portion of the circular flat space 42 this protruding portion of the porous protective layer 86 functions as a filter which serves to trap particulate substances which would otherwise cause an undesirable clogging or plugging of the thin flat space 42 and adversely affect the output of the sensor according to the instant arrangement the diffusion of the measurement gas toward the first and second portions of the flat space 42 is restricted first by the porous body 86 and then by the small thickness of the flat space 42 in this sense the present arrangement is considered to provide a theoretically ideal state of diffusion of the measurement gas toward the electrodes 56 58 60 62 the embodiment shown in fig1 13 is different from the embodiments of fig8 and 10 in that the thin flat space 42 has a rectangular shape as viewed perpendicularly to the plane of the sensing element as indicated in fig1 the four corner portions of the rectangular flat space 42 serve as the second portion in each of the four corner portions the second inner pumping electrode 58 and the second measuring electrode 62 are disposed so as to face each other as most clearly shown in fig1 adjacent to the central portion of the rectangular flat space 42 communicating with the gas inlet aperture 44 there is disposed the first portion in which the annular first inner pumping electrode 56 and the annular first measuring electrode 60 are disposed radially outwardly of the first measuring electrode 60 there is formed partition means in the.