A spark plug comprising: a tubular insulator having an axial bore extending through the insulator in a direction of an axis; a center electrode inserted into a forward end portion of the axial bore; a tubular metallic shell provided externally of an outer circumference of the insulator; a ground electrode disposed at a forward end portion of the metallic shell; and a noble metal tip joined to a forward end portion of the center electrode and forming a gap in cooperation with the ground electrode; the center electrode having a shoulder portion at a forward end portion of the center electrode, the shoulder portion tapering forward with respect to the direction of the axis, the noble metal tip being jointed to the center electrode by means of a fusion zone being formed at least partially at a proximal end portion of the noble metal tip, and an outside diameter of the fusion zone as measured at a distal end of the fusion zone is smaller than that as measured at a proximal end of the fusion zone, the spark plug being characterized in that: with θ1 representing an acute angle formed by a straight line L1 and a straight line L2 defined below, a relational expression θ1 ≤ 72° is satisfied, and with θ2 representing an acute angle formed by a straight line L3 and a straight line L4 defined below, as viewed on a section which contains the axis and on which θ2 is maximized, relational expressions θ1 > θ2 and (θ1 - θ2) ≤ 50° are satisfied, where the straight line L1 is, as viewed on a section which contains the axis, an axially forward extended straight line of one of two outlines (OL1, OL2) of the shoulder portion, the two outlines (OL1, OL2) being located on opposite sides with respect to the axis, the straight line L2 is, as viewed on the section which contains the axis, an axially forward extended straight line of the other of the two outlines (OL1, OL2) of the shoulder portion, the two outlines being located on opposite sides with respect to the axis, the straight line L3 is, as viewed on the section which contains the axis, a straight line which passes through opposite ends of one of two outlines (OL3, OL4)of an externally exposed surface of the fusion zone, the one outline being located on one side with respect to the axis, and the straight line L4 is, as viewed on the section which contains the axis, a straight line which passes through opposite ends of the other of the two outlines (OL3, OL4) of the externally exposed surface of the fusion zone, the other outline being located on the other side with respect to the axis, a shortest distance between the fusion zone and a distal end surface of the noble metal tip is 0.8 mm to 1.2 mm inclusive as measured on an outer side surface of the noble metal tip along the axis. a tubular insulator having an axial bore extending through the insulator in a direction of an axis; a center electrode inserted into a forward end portion of the axial bore; a tubular metallic shell provided externally of an outer circumference of the insulator; a ground electrode disposed at a forward end portion of the metallic shell; and a noble metal tip joined to a forward end portion of the center electrode and forming a gap in cooperation with the ground electrode; the center electrode having a shoulder portion at a forward end portion of the center electrode, the shoulder portion tapering forward with respect to the direction of the axis, the noble metal tip being jointed to the center electrode by means of a fusion zone being formed at least partially at a proximal end portion of the noble metal tip, and an outside diameter of the fusion zone as measured at a distal end of the fusion zone is smaller than that as measured at a proximal end of the fusion zone, the spark plug being characterized in that: with θ1 representing an acute angle formed by a straight line L1 and a straight line L2 defined below, a relational expression θ1 ≤ 72° is satisfied, and with θ2 representing an acute angle formed by a straight line L3 and a straight line L4 defined below, as viewed on a section which contains the axis and on which θ2 is maximized, relational expressions θ1 > θ2 and (θ1 - θ2) ≤ 50° are satisfied, where the straight line L1 is, as viewed on a section which contains the axis, an axially forward extended straight line of one of two outlines (OL1, OL2) of the shoulder portion, the two outlines (OL1, OL2) being located on opposite sides with respect to the axis, the straight line L2 is, as viewed on the section which contains the axis, an axially forward extended straight line of the other of the two outlines (OL1, OL2) of the shoulder portion, the two outlines being located on opposite sides with respect to the axis, the straight line L3 is, as viewed on the section which contains the axis, a straight line which passes through opposite ends of one of two outlines (OL3, OL4)of an externally exposed surface of the fusion zone, the one outline being located on one side with respect to the axis, and the straight line L4 is, as viewed on the section which contains the axis, a straight line which passes through opposite ends of the other of the two outlines (OL3, OL4) of the externally exposed surface of the fusion zone, the other outline being located on the other side with respect to the axis, a shortest distance between the fusion zone and a distal end surface of the noble metal tip is 0.8 mm to 1.2 mm inclusive as measured on an outer side surface of the noble metal tip along the axis.