An internal combustion engine fitted with a combustion pressure detection device includes: an internal combustion engine having a combustion chamber, a pressure detection device having a housing, a diaphragm, and a detection member, and having a shoulder part. The diaphragm is provided on the front end side of the housing and the detection member arranged inside the housing and behind the diaphragm, allowing it to detect the pressure working via the diaphragm. Also provided is a second seal member in a ring shape that seals the opening in the combustion chamber and the housing in the shoulder part of the housing in the pressure detection device.