A spark plug comprising: a center electrode; a metal shell that holds therein the center electrode insulatedly; and a ground electrode (30, 120, 130, 140, 150) that includes an electrode base having a facing surface facing the center electrode and a tip (34, 121, 131, 141, 151) containing a noble metal and arranged on the facing surface of the electrode base, the electrode base being joined at a first end portion thereof to the metal shell, wherein the electrode base has: an opposite surface located opposite the facing surface; an end surface connecting the opposite surface and the facing surface at a second end portion of the electrode base opposite the first end portion; and a pair of side surfaces continuing to the end surface via sides of the second end portion and connecting the facing surface and the opposite surface, wherein the tip (34, 121, 131, 141, 151) has: a top surface (90, 122, 132, 142, 152) facing the center electrode; and a bottom surface (95, 125, 134, 144, 154) located opposite the top surface (90, 122, 132, 142, 152) and joined to the electrode base with a weld zone formed therebetween, wherein the weld zone has: a back region exposed at the opposite surface of the electrode base; a joint region at which the tip (34, 121, 131, 141, 151) is joined; and a connection region connecting the joint region and the back region in a thickness direction of the electrode base without being exposed at the side surfaces of the electrode base, and characterised in that, in a cross section of the ground electrode (30, 120, 130, 140, 150) taken through the side surfaces of the electrode base along a plane passing through a center of the top surface (90, 122, 132, 142, 152) and a center of the bottom surface (95, 125, 134, 144, 154) of the tip (34, 121, 131, 141, 151), a maximum width of the connection region in a direction perpendicular to the thickness direction of the electrode base is larger than a width of the back region, and a value (W3/W4) of a width of the top surface (90, 122, 132, 142, 152) of the tip (34, 121, 131, 141, 151) being divided by a width of the facing surface of the electrode base is greater than 0.3. a center electrode; a metal shell that holds therein the center electrode insulatedly; and a ground electrode (30, 120, 130, 140, 150) that includes an electrode base having a facing surface facing the center electrode and a tip (34, 121, 131, 141, 151) containing a noble metal and arranged on the facing surface of the electrode base, the electrode base being joined at a first end portion thereof to the metal shell, wherein the electrode base has: an opposite surface located opposite the facing surface; an end surface connecting the opposite surface and the facing surface at a second end portion of the electrode base opposite the first end portion; and a pair of side surfaces continuing to the end surface via sides of the second end portion and connecting the facing surface and the opposite surface, wherein the tip (34, 121, 131, 141, 151) has: a top surface (90, 122, 132, 142, 152) facing the center electrode; and a bottom surface (95, 125, 134, 144, 154) located opposite the top surface (90, 122, 132, 142, 152) and joined to the electrode base with a weld zone formed therebetween, wherein the weld zone has: a back region exposed at the opposite surface of the electrode base; a joint region at which the tip (34, 121, 131, 141, 151) is joined; and a connection region connecting the joint region and the back region in a thickness direction of the electrode base without being exposed at the side surfaces of the electrode base, and characterised in that, in a cross section of the ground electrode (30, 120, 130, 140, 150) taken through the side surfaces of the electrode base along a plane passing through a center of the top surface (90, 122, 132, 142, 152) and a center of the bottom surface (95, 125, 134, 144, 154) of the tip (34, 121, 131, 141, 151), a maximum width of the connection region in a direction perpendicular to the thickness direction of the electrode base is larger than a width of the back region, and a value (W3/W4) of a width of the top surface (90, 122, 132, 142, 152) of the tip (34, 121, 131, 141, 151) being divided by a width of the facing surface of the electrode base is greater than 0.3.