Internal combustion engine fitted with combustion pressure detection device

An internal combustion engine fitted with a combustion pressure detection device, comprising:nthe internal combustion engine including a combustion chamber and an opening formed on a wall surface constituting the combustion chamber, the opening connecting to the combustion chamber and having a wider diameter on a near side than on a far side thereof;the combustion pressure detection device including: a housing having a cylindrical shape that is insertable into the opening of the internal combustion engine, the housing being narrower on a front end side thereof in an insertion direction in accordance with the diameter of the opening on the far side, and being wider on a rear end side thereof in accordance with the diameter of the opening on the near side, and the housing having a shoulder part between the front end side and the rear end side thereof; a diaphragm provided on the front end side of the housing in the insertion direction; and a detection member arranged inside the housing and behind the diaphragm, and detecting the pressure working via the diaphragm; anda seal member in a ring shape that has a cross section in an opening shape, the seal member sealing the opening in the combustion chamber and the housing at the shoulder part of the housing in the combustion pressure detection device, whereinthe housing includes a step part on the front end side in the insertion direction relative to the shoulder part, andthe seal member is held by the housing by latching an opening edge part of an opening part in the opening shape onto the step part.