Gas sensor and casing for containing sensor element

A sensor element containment casing for containing a sensor element and a connector while securing the sensor element therein, the sensor element including, on a side of one end portion thereof, a sensing part for sensing a predetermined gas component contained in a measurement gas, the connector electrically connecting the sensor element to an outside, the sensor element containment casing comprising:

• an outer tube including a main portion in which a reference gas is included and a sealing portion being an end portion having a smaller diameter than the main portion, another end portion of the sensor element protruding to the main portion;
• a rubber seal member fitted into the sealing portion to seal the outer tube;
• and
• a ceramic spacer intervening between the seal member and the connector in the outer tube, wherein
• the ceramic spacer includes a concave portion in an end surface on a side having contact with the connector, and has contact with the connector in the end surface except for the concave portion, and
• when a smaller one of an area of a contact surface of the connector having contact with the ceramic spacer and an area of the whole end surface including the concave portion of the spacer is S0, and a contact area between the connector and the ceramic spacer is S. satisfied for a contact portion area ratio S/S0 is:
• 0.2≤S/S0≤0.7.