A sensor includes a sensor element; a circuit board having an element attachment surface on which the sensor element is mounted; a casing member for housing the circuit board, the casing member having a measurement chamber which faces the sensor element and communicates with the target atmosphere through a gas inlet; an annular elastic seal member which is interposed between the element attachment surface and the casing member in such a manner that the elastic seal member is in contact with the element attachment surface. The elastic seal member is made of foamed rubber and has an end surface flat portion which faces the casing member and another end surface flat portion which faces the element attachment. The elastic seal member is interposed between the element attachment surface and the casing member such that the elastic seal member is compressed between the flat portions of the end surfaces.