A NOx sensor composed of an oxygen ion conductive solid electrolyte (1, 2, 3, 4, 5, 6), in which a NOx concentration in a measurement gas is specified on the basis of a current flowing in said solid electrolyte by decomposition of NOx, said NOx sensor comprising: an internal space (20, 40) to which the measurement gas is introduced from outside; a reference gas space to which a reference gas is introduced; at least one pumping cell (21, 50) including a first electrode (22, 51) and a second electrode and pumping out oxygen in said internal space (20, 40) by applying a predetermined voltage between said first electrode and said second electrode; and a measuring cell including a third electrode and a fourth electrode and 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; wherein said first electrode (22, 51), which is provided on a surface of said internal space (20, 40) is formed of porous cermet consisting of a noble metal and an oxygen ion conductive solid electrolyte, said second electrode is formed in a different space from said internal space; said third electrode is formed on the surface of said internal space (20, 40), and said fourth electrode is formed in a different portion from said internal space (20, 40); wherein the porosity of said first electrode is greater than or equal to 15% and less than or equal to 40%; and wherein said internal space (20, 40) includes: a first internal space to which said measurement gas is introduced from said outside, and a second internal space formed to be in communication with said first internal space under a predetermined diffusion resistance, said at least one pumping cell includes: a main pumping cell having its said first electrode in said first internal space, and an auxiliary pumping cell having its said first electrode in said second internal space, said third electrode is formed in said second internal space, said second electrode is shared by said main pumping cell and said auxiliary pumping cell, and the porosity of each said first electrode (22, 51) is greater than or equal to 15% and less than or equal to 40%. an internal space (20, 40) to which the measurement gas is introduced from outside; a reference gas space to which a reference gas is introduced; at least one pumping cell (21, 50) including a first electrode (22, 51) and a second electrode and pumping out oxygen in said internal space (20, 40) by applying a predetermined voltage between said first electrode and said second electrode; and a measuring cell including a third electrode and a fourth electrode and 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; wherein said first electrode (22, 51), which is provided on a surface of said internal space (20, 40) is formed of porous cermet consisting of a noble metal and an oxygen ion conductive solid electrolyte, said second electrode is formed in a different space from said internal space; said third electrode is formed on the surface of said internal space (20, 40), and said fourth electrode is formed in a different portion from said internal space (20, 40); wherein the porosity of said first electrode is greater than or equal to 15% and less than or equal to 40%; and wherein said internal space (20, 40) includes: a first internal space to which said measurement gas is introduced from said outside, and a second internal space formed to be in communication with said first internal space under a predetermined diffusion resistance, a first internal space to which said measurement gas is introduced from said outside, and a second internal space formed to be in communication with said first internal space under a predetermined diffusion resistance, said at least one pumping cell includes: a main pumping cell having its said first electrode in said first internal space, and an auxiliary pumping cell having its said first electrode in said second internal space, said third electrode is formed in said second internal space, said second electrode is shared by said main pumping cell and said auxiliary pumping cell, and the porosity of each said first electrode (22, 51) is greater than or equal to 15% and less than or equal to 40%. a main pumping cell having its said first electrode in said first internal space, and an auxiliary pumping cell having its said first electrode in said second internal space, said third electrode is formed in said second internal space, said second electrode is shared by said main pumping cell and said auxiliary pumping cell, and the porosity of each said first electrode (22, 51) is greater than or equal to 15% and less than or equal to 40%.