A spark plug comprising: a center electrode ; a metal shell that holds therein the center electrode insulatedly; and a ground electrode that includes an electrode base having a facing surface facing the center electrode and a tip 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 has: a top surface facing the center electrode ; and a bottom surface located opposite the top surface 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 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 taken through the side surfaces of the electrode base along a plane passing through a center of the top surface and a center of the bottom surface of the tip , a maximum width (W1) of the connection region in a direction perpendicular to the thickness direction of the electrode base is larger than a width (W2) of the back region , and a value (W3/W4) of a width (W3) of the top surface of the tip being divided by a width (W4) 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 that includes an electrode base having a facing surface facing the center electrode and a tip 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 has: a top surface facing the center electrode ; and a bottom surface located opposite the top surface 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 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 taken through the side surfaces of the electrode base along a plane passing through a center of the top surface and a center of the bottom surface of the tip , a maximum width (W1) of the connection region in a direction perpendicular to the thickness direction of the electrode base is larger than a width (W2) of the back region , and a value (W3/W4) of a width (W3) of the top surface of the tip being divided by a width (W4) of the facing surface of the electrode base is greater than 0.3. <EOS>