A spark plug has a pressure detector which includes a diaphragm and semiconductor strain gauges. The diaphragm is provided in the vicinity of the combustion chamber within the housing and extending substantially in the axial direction of the housing to thereby form a reference chamber separated away from the combustion chamber and to thereby generate a strain due to a differential pressure between a pressure within the combustion chamber and a pressure within the reference chamber. The semiconductor strain gauges are formed in the diaphragm for changing its electrical signal in accordance with the strain of the diaphragm. <IMAGE> <IMAGE>
