Sensor

A sensor comprising:

• a sensor element extending in an axial direction, having a periphery and comprising a leading end portion exposed to a gas measurement object;
• a metal shell covering the periphery of the sensor element and comprising a rear end side;
• an outer cylinder member fixed to the rear end side of the metal shell, the outer cylinder member being composed of a first metal material, having an outer surface and comprising a leading end side;
• and
• a protection outer cylinder member covering the outer surface of the outer cylinder member, the protection outer cylinder being composed of a second metal material and having a circumference;
• wherein the outer cylinder member further comprises a sealing part in contact with the circumference of the protection outer cylinder member, and a spaced part positioned closer to the leading end side than the sealing part, opposed to the protection outer cylinder member, and spaced from the protection outer cylinder member;
• and
• wherein the first metal material is less subject to rusting than the second metal material.