A sensor comprising: a sensor element; a metal shell which holds said sensor element; one or more sensor terminal members (11, 12) which are electrically connected to said sensor element, and which elongate from said sensor element toward a rear end; a metal outer tube which is connected to said metal shell at a tip end portion of said metal outer tube itself; and an electrically insulative separator which is housed inside said metal outer tube, in which said sensor terminal members (11, 12) are positioned, and which provides at least one of insulation between said sensor terminal members (11,12), and insulation between said sensor terminal members (11,12) and said metal outer tube, wherein said separator has: a rear end side portion; and a flange portion which is positioned on a tip end side with respect to said rear end side portion, and which is larger in diameter than said rear end side portion, said metal outer tube comprising a step portion or an inner projection which butts against said flange portion of said separator, at least one of an outer-tube butting face of said flange portion of said separator and a flange butting face of said step portion or said inner projection of said metal outer tube forms an inclined face which is more radially outward positioned as further advancing toward said tip end, said outer-tube butting face butting against said step portion or said inner projection of said metal outer tube and being positioned on a side of said rear end, said flange butting face butting against said outer-tube butting face of said flange portion of said separator, and said separator is held by said metal outer tube in a state where said separator is pressed toward said rear end; characterised in that: a gap is present between the flange portion of the separator and a flange surrounding portion of the metal outer tube which is positioned around the flange portion; said sensor has a pressing member which holds said separator while axially rearwardly pressing said separator against said metal outer tube; said metal outer tube has a deformed portion which holds said pressing member while radially inward pressing said pressing member; said separator has, between said step portion portion or inner projection and said deformed portion, said flange portion which butts at a rear end side against said metal outer tube while butting at a tip end side against said pressing member. a sensor element; a metal shell which holds said sensor element; one or more sensor terminal members (11, 12) which are electrically connected to said sensor element, and which elongate from said sensor element toward a rear end; a metal outer tube which is connected to said metal shell at a tip end portion of said metal outer tube itself; and an electrically insulative separator which is housed inside said metal outer tube, in which said sensor terminal members (11, 12) are positioned, and which provides at least one of insulation between said sensor terminal members (11,12), and insulation between said sensor terminal members (11,12) and said metal outer tube, wherein said separator has: a rear end side portion; and a flange portion which is positioned on a tip end side with respect to said rear end side portion, and which is larger in diameter than said rear end side portion, said metal outer tube comprising a step portion or an inner projection which butts against said flange portion of said separator, at least one of an outer-tube butting face of said flange portion of said separator and a flange butting face of said step portion or said inner projection of said metal outer tube forms an inclined face which is more radially outward positioned as further advancing toward said tip end, said outer-tube butting face butting against said step portion or said inner projection of said metal outer tube and being positioned on a side of said rear end, said flange butting face butting against said outer-tube butting face of said flange portion of said separator, and said separator is held by said metal outer tube in a state where said separator is pressed toward said rear end; characterised in that: a gap is present between the flange portion of the separator and a flange surrounding portion of the metal outer tube which is positioned around the flange portion; said sensor has a pressing member which holds said separator while axially rearwardly pressing said separator against said metal outer tube; said metal outer tube has a deformed portion which holds said pressing member while radially inward pressing said pressing member; said separator has, between said step portion portion or inner projection and said deformed portion, said flange portion which butts at a rear end side against said metal outer tube while butting at a tip end side against said pressing member. a gap is present between the flange portion of the separator and a flange surrounding portion of the metal outer tube which is positioned around the flange portion; said sensor has a pressing member which holds said separator while axially rearwardly pressing said separator against said metal outer tube; said metal outer tube has a deformed portion which holds said pressing member while radially inward pressing said pressing member; said separator has, between said step portion portion or inner projection and said deformed portion, said flange portion which butts at a rear end side against said metal outer tube while butting at a tip end side against said pressing member.