A spark plug comprising: an insulator having an axial hole extending in a direction of an axis; a center electrode held at one end of the axial hole; a metallic terminal held at the other end of the axial hole; and a connecting portion which electrically connects the center electrode and the metallic terminal within the axial hole, the spark plug being characterized in that the metallic terminal has a first constituent portion exposed from the axial hole, and a second constituent portion accommodated in the axial hole; the side of the metallic terminal where the first constituent portion is provided is defined as the rear end side with respect to the direction of the axis O; the axial hole has an intermediate diameter portion in which a forward end portion of the second constituent portion is disposed; and when a diameter of the forward end portion of the second constituent portion is referred to as a forward end portion diameter, a diameter of the intermediate diameter portion is referred to as an intermediate diameter portion diameter, a length from the rear end of the center electrode to the rear end of a connecting member constituting the connecting portion is referred to as a charging length, and a length from the rear end of the center electrode to the forward end of the second constituent portion is referred to as a connecting portion length, a distance ((B-A)/2) between the forward end portion of the second constituent portion and the wall surface of the intermediate diameter portion falls within a range of 0.02 mm to 0.2 mm, a shrinkage amount (D-C) falls within a range of 6 mm to 27 mm, and the second constituent portion has a Vickers hardness of 100 Hv to 430 Hv at ordinary temperature. an insulator having an axial hole extending in a direction of an axis; a center electrode held at one end of the axial hole; a metallic terminal held at the other end of the axial hole; and a connecting portion which electrically connects the center electrode and the metallic terminal within the axial hole, the spark plug being characterized in that the metallic terminal has a first constituent portion exposed from the axial hole, and a second constituent portion accommodated in the axial hole; the side of the metallic terminal where the first constituent portion is provided is defined as the rear end side with respect to the direction of the axis O; the axial hole has an intermediate diameter portion in which a forward end portion of the second constituent portion is disposed; and when a diameter of the forward end portion of the second constituent portion is referred to as a forward end portion diameter, a diameter of the intermediate diameter portion is referred to as an intermediate diameter portion diameter, a length from the rear end of the center electrode to the rear end of a connecting member constituting the connecting portion is referred to as a charging length, and a length from the rear end of the center electrode to the forward end of the second constituent portion is referred to as a connecting portion length, a distance ((B-A)/2) between the forward end portion of the second constituent portion and the wall surface of the intermediate diameter portion falls within a range of 0.02 mm to 0.2 mm, a shrinkage amount (D-C) falls within a range of 6 mm to 27 mm, and the second constituent portion has a Vickers hardness of 100 Hv to 430 Hv at ordinary temperature.