Abstract: A combustion process sensor for detecting combustion processes occurring in a combustion chamber of an engine has a generally tubular mounting casing to be screwed into a threaded hole in the combustion chamber wall and a quartz-glass rod axially extending in the mounting casing and held in position therein. The casing includes two axially aligned generally tubular members rigidly secured together at their own ends. The quartz-glass rod has a first portion remote from a sensing end thereof adapted to be exposed to combustion flame in the combustion chamber, and a second portion between the first portion and the sensing end. The first portion is secured to one of the two tubular members of the casing, while the second portion is supported from the casing only by a vibration damping material disposed in engagement with the outer peripheral surface of the quartz-glass rod and the inner peripheral surface of the casing.