Glow plug with combustion pressure detecting function

A glow plug comprising:

• a cylindrical housing;
• a heater member held in said housing for generating heat upon energization;
• a conductive center pole inserted into said housing and conducted to said heater member for being displaced relative to said housing in an axial direction along an axis of said housing in accordance with a change in a combustion pressure of an internal combustion engine;
• a first piezoelectric element for generating a charge in response to a change in a stress applied to said first piezoelectric element, said first piezoelectric element having a first plus face for generating a plus charge and a first minus face for generating a minus charge when said first piezoelectric element is compressed in an axial direction;
• and
• a second piezoelectric element for generating a charge in response to a change in a stress applied to said second piezoelectric element, said second piezoelectric element having a second plus face for generating a plus charge and a second minus face for generating a minus charge when said second piezoelectric element is compressed in an axial direction;
• wherein each of said first piezoelectric element and said second piezoelectric element is subjected to a preload for compressing it in said axial direction and is arranged such that, when said center pole is displaced to a root-end side, a load to be applied to one of said first piezoelectric element and said second piezoelectric element for compressing the same in said axial direction increases whereas a load to be applied to the other for compressing the same in said axial direction decreases.