A combustion pressure sensor (5, 5A) comprising at least: a case member which has a joint surface on a side of a tip end and in which an opening portion is provided; a pressure reception member on which a combustion pressure acts, which has a joint surface which is brought into planar contact with the joint surface of the case member to block the opening portion; a welding portion in which the joint surface of the case member and the joint surface of the pressure reception member are joined to each other, the welding portion being provided by a state where the joint surface of the pressure reception member makes contact with the joint surface of the case member; and a piezoelectric element which is stored in the case member so as to receive the combustion pressure transmitted from the pressure reception member, and which converts a pressing force based on the combustion pressure transmitted from the pressure reception member into a signal, wherein a predetermined clearance portion is provided so as to form a predetermined gap between the joint surface of the case member and the joint surface of the pressure reception member, wherein the joint surface of the case member and the joint surface of the pressure reception member are not joined to each other in the predetermined clearance portion, wherein a protrusion portion is provided by being protruded and formed from the case member so as to be located between the predetermined clearance portion and the opening portion and cover the predetermined clearance portion, wherein the predetermined gap formed by the predetermined clearance portion is formed by the joint surface and a step portion which is formed on the pressure reception member, and wherein the welding portion is located on an opposite side of the predetermined clearance portion to the protrusion portion. a case member which has a joint surface on a side of a tip end and in which an opening portion is provided; a pressure reception member on which a combustion pressure acts, which has a joint surface which is brought into planar contact with the joint surface of the case member to block the opening portion; a welding portion in which the joint surface of the case member and the joint surface of the pressure reception member are joined to each other, the welding portion being provided by a state where the joint surface of the pressure reception member makes contact with the joint surface of the case member; and a piezoelectric element which is stored in the case member so as to receive the combustion pressure transmitted from the pressure reception member, and which converts a pressing force based on the combustion pressure transmitted from the pressure reception member into a signal, wherein a predetermined clearance portion is provided so as to form a predetermined gap between the joint surface of the case member and the joint surface of the pressure reception member, wherein the joint surface of the case member and the joint surface of the pressure reception member are not joined to each other in the predetermined clearance portion, wherein a protrusion portion is provided by being protruded and formed from the case member so as to be located between the predetermined clearance portion and the opening portion and cover the predetermined clearance portion, wherein the predetermined gap formed by the predetermined clearance portion is formed by the joint surface and a step portion which is formed on the pressure reception member, and wherein the welding portion is located on an opposite side of the predetermined clearance portion to the protrusion portion.