An internal combustion engine fitted with a combustion pressure detection device, comprising: an 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 far side from the combustion chamber than on a near side from the combustion chamber; a 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 which is on the near side from the combustion chamber, and being wider on a rear end side thereof in the insertion direction which is on the far side from the combustion chamber, 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; a seal member (72, 272, 372) in a ring shape, the seal member (72, 272, 372) sealing the opening in the combustion chamber and the housing at the shoulder part of the housing of the combustion pressure detection device, characterized by the seal member (72, 272, 372) being formed to have a hollow interior and has an opening part (72t, 272t, 372t) that is an annular incision formed over an entire periphery in a circumferential direction on one end side in the center axis direction, wherein the housing includes a step part, which is a groove formed over the entire region in the circumferential direction on the front end side in the insertion direction relative to the shoulder part, and the seal member (72, 272, 372), whose opening part (72t, 272t, 372t) faces the combustion chamber, is held and attached by the housing by fitting one end of an opening edge part of the opening part (72t, 272t, 372t) into the step part. an 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 far side from the combustion chamber than on a near side from the combustion chamber; a 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 which is on the near side from the combustion chamber, and being wider on a rear end side thereof in the insertion direction which is on the far side from the combustion chamber, 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; 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 which is on the near side from the combustion chamber, and being wider on a rear end side thereof in the insertion direction which is on the far side from the combustion chamber, 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; a seal member (72, 272, 372) in a ring shape, the seal member (72, 272, 372) sealing the opening in the combustion chamber and the housing at the shoulder part of the housing of the combustion pressure detection device, characterized by the seal member (72, 272, 372) being formed to have a hollow interior and has an opening part (72t, 272t, 372t) that is an annular incision formed over an entire periphery in a circumferential direction on one end side in the center axis direction, wherein the housing includes a step part, which is a groove formed over the entire region in the circumferential direction on the front end side in the insertion direction relative to the shoulder part, and the seal member (72, 272, 372), whose opening part (72t, 272t, 372t) faces the combustion chamber, is held and attached by the housing by fitting one end of an opening edge part of the opening part (72t, 272t, 372t) into the step part.