A spark plug, comprising: a first electrode (100; 400); and a second electrode (400; 401; 402; 403; 404; 405; 406; 407; 408; 409; 100) including an electrode base (410; 110) and an electrode tip (450; 450A; 450B; 450C; 450D; 150) joined to the electrode base (410; 110) so as to define a gap between the first electrode (100; 400) and the electrode tip (450; 450A; 450B; 450C; 450D; 150), wherein the electrode tip (450; 450A; 450B; 450C; 450D; 150) has a flat surface (451; 151) located apart from the electrode base (410; 110) and is joined to the electrode base (410; 110) at a side of the electrode tip (450; 450A; 450B; 450C; 450D; 150) opposite to the flat surface (451; 151) by laser welding with emission of a laser beam in a plane direction from one end to the other end of the flat surface (451; 151); wherein the spark plug comprises a fused part (430; 430D; 130) formed by the laser welding at the opposite side of the electrode tip (450; 450A; 450B; 450C; 450D; 150); and wherein the fused part (430; 430D; 130) extends in a plane direction from one end to the other end of the flat surface (451; 151) and has an exposed surface (431; 131) exposed at one end side of the second electrode (400; 401; 402; 403; 404; 405; 406; 407; 408; 100) and formed on the area of emission of the laser beam and an end region located furthest apart from the exposed surface (431; 131); and wherein a cross-sectional shape of the fused part (430; 430D; 130) taken along a plane perpendicular to the flat surface (451; 151) and parallel to the plane direction has a constricted section (432; 132) at a position from one side to the other side in the plane direction, and, except for constricted section (432; 132), tapers down from the exposed surface (431; 131) to the end region in the emission direction LD of the laser beam. a first electrode (100; 400); and a second electrode (400; 401; 402; 403; 404; 405; 406; 407; 408; 409; 100) including an electrode base (410; 110) and an electrode tip (450; 450A; 450B; 450C; 450D; 150) joined to the electrode base (410; 110) so as to define a gap between the first electrode (100; 400) and the electrode tip (450; 450A; 450B; 450C; 450D; 150), wherein the electrode tip (450; 450A; 450B; 450C; 450D; 150) has a flat surface (451; 151) located apart from the electrode base (410; 110) and is joined to the electrode base (410; 110) at a side of the electrode tip (450; 450A; 450B; 450C; 450D; 150) opposite to the flat surface (451; 151) by laser welding with emission of a laser beam in a plane direction from one end to the other end of the flat surface (451; 151); wherein the spark plug comprises a fused part (430; 430D; 130) formed by the laser welding at the opposite side of the electrode tip (450; 450A; 450B; 450C; 450D; 150); and wherein the fused part (430; 430D; 130) extends in a plane direction from one end to the other end of the flat surface (451; 151) and has an exposed surface (431; 131) exposed at one end side of the second electrode (400; 401; 402; 403; 404; 405; 406; 407; 408; 100) and formed on the area of emission of the laser beam and an end region located furthest apart from the exposed surface (431; 131); and wherein a cross-sectional shape of the fused part (430; 430D; 130) taken along a plane perpendicular to the flat surface (451; 151) and parallel to the plane direction has a constricted section (432; 132) at a position from one side to the other side in the plane direction, and, except for constricted section (432; 132), tapers down from the exposed surface (431; 131) to the end region in the emission direction LD of the laser beam.