A sensor element constituted by an oxygen-ion conductive solid electrolyte as a main component, said sensor element comprising: an internal space (20, 40) to which the measurement gas is introduced from the outside; a diffusion control part in communication with said internal space (20, 40) in a lengthwise direction of said sensor element, said diffusion control part being one horizontally long slit with a smaller thickness than that of said internal space (20, 40), said diffusion control part applying a predetermined diffusion resistance to the measurement gas having been introduced to said internal space (20, 40), said diffusion control part having an upper portion which is flat; a pumping cell (21, 50) that pumps out oxygen existing in said internal space (20, 40); and a measuring cell, said pumping cell (21, 50) comprising: a first electrode (22, 51) formed on a surface of said internal space (20, 40); and a second electrode formed in a space different from said internal space (20, 40), said pumping cell (21, 50) pumping out oxygen existing in said internal space (20, 40) when a predetermined voltage (VpO, Vp1) is applied between said first electrode (22, 51) and said second electrode, said measuring cell comprising: a third electrode formed in said diffusion control part, said third electrode reducing an oxide gas component in said predetermined gas component to which said predetermined diffusion resistance has been applied by said diffusion control part, wherein said third electrode is exposed in said diffusion control part; and a fourth electrode formed in a part different from said diffusion control part, said measuring cell measuring a current flowing between said third electrode and said fourth electrode when a voltage is applied between said third electrode and said fourth electrode. an internal space (20, 40) to which the measurement gas is introduced from the outside; a diffusion control part in communication with said internal space (20, 40) in a lengthwise direction of said sensor element, said diffusion control part being one horizontally long slit with a smaller thickness than that of said internal space (20, 40), said diffusion control part applying a predetermined diffusion resistance to the measurement gas having been introduced to said internal space (20, 40), said diffusion control part having an upper portion which is flat; a pumping cell (21, 50) that pumps out oxygen existing in said internal space (20, 40); and a measuring cell, said pumping cell (21, 50) comprising: a first electrode (22, 51) formed on a surface of said internal space (20, 40); and a second electrode formed in a space different from said internal space (20, 40), said pumping cell (21, 50) pumping out oxygen existing in said internal space (20, 40) when a predetermined voltage (VpO, Vp1) is applied between said first electrode (22, 51) and said second electrode, said measuring cell comprising: a third electrode formed in said diffusion control part, said third electrode reducing an oxide gas component in said predetermined gas component to which said predetermined diffusion resistance has been applied by said diffusion control part, wherein said third electrode is exposed in said diffusion control part; and a fourth electrode formed in a part different from said diffusion control part, said measuring cell measuring a current flowing between said third electrode and said fourth electrode when a voltage is applied between said third electrode and said fourth electrode. a first electrode (22, 51) formed on a surface of said internal space (20, 40); and a second electrode formed in a space different from said internal space (20, 40), said pumping cell (21, 50) pumping out oxygen existing in said internal space (20, 40) when a predetermined voltage (VpO, Vp1) is applied between said first electrode (22, 51) and said second electrode, said measuring cell comprising: a third electrode formed in said diffusion control part, said third electrode reducing an oxide gas component in said predetermined gas component to which said predetermined diffusion resistance has been applied by said diffusion control part, wherein said third electrode is exposed in said diffusion control part; and a fourth electrode formed in a part different from said diffusion control part, said measuring cell measuring a current flowing between said third electrode and said fourth electrode when a voltage is applied between said third electrode and said fourth electrode. a third electrode formed in said diffusion control part, said third electrode reducing an oxide gas component in said predetermined gas component to which said predetermined diffusion resistance has been applied by said diffusion control part, wherein said third electrode is exposed in said diffusion control part; and a fourth electrode formed in a part different from said diffusion control part, said measuring cell measuring a current flowing between said third electrode and said fourth electrode when a voltage is applied between said third electrode and said fourth electrode.