A spark plug comprising: a tubular insulator having an axial hole extending in a direction of an axis; a center electrode inserted into a forward end portion of the axial hole; a tubular metallic shell provided around the insulator; a ground electrode provided at a forward end portion of the metallic shell; and a tip which is made of metal, whose base end portion is joined to a forward end portion of the center electrode and whose forward end portion forms a gap in cooperation with a distal end portion of the ground electrode, characterized in that the tip is joined to the center electrode through a fusion portion which is formed over the entire circumference of the interface between a peripheral portion of a base end of the tip and a peripheral portion of a forward end of the center electrode and in which the tip and the center electrode are fused and mixed together, the center electrode has a coefficient of linear thermal expansion greater than that of the tip, the tip has a gap-forming portion which has a maximum width of 1.2 mm or greater as measured on a cross section including the axis and which forms the gap in cooperation with the ground electrode, and a to-be-joined portion which is located between the gap-forming portion and the fusion portion and adjacent to the fusion portion and which is joined to the center electrode, at a position A which is shifted 0.1 mm from a forward end of an outer surface of the fusion portion toward the forward end side with respect to the direction of the axis, the width of the to-be-joined portion, measured on the cross section including the axis, is made smaller than the width of the gap-forming portion measured on the cross section, and the to-be-joined portion satisfies a relation Dtw/Dw ≤ 1.1, where Dtw represents the width (mm) of the to-be-joined portion at the position A measured on the cross section, and Dw represents the width (mm) of the fusion portion at the forward end of the outer surface thereof measured on the cross section. a tubular insulator having an axial hole extending in a direction of an axis; a center electrode inserted into a forward end portion of the axial hole; a tubular metallic shell provided around the insulator; a ground electrode provided at a forward end portion of the metallic shell; and a tip which is made of metal, whose base end portion is joined to a forward end portion of the center electrode and whose forward end portion forms a gap in cooperation with a distal end portion of the ground electrode, characterized in that the tip is joined to the center electrode through a fusion portion which is formed over the entire circumference of the interface between a peripheral portion of a base end of the tip and a peripheral portion of a forward end of the center electrode and in which the tip and the center electrode are fused and mixed together, the center electrode has a coefficient of linear thermal expansion greater than that of the tip, the tip has a gap-forming portion which has a maximum width of 1.2 mm or greater as measured on a cross section including the axis and which forms the gap in cooperation with the ground electrode, and a to-be-joined portion which is located between the gap-forming portion and the fusion portion and adjacent to the fusion portion and which is joined to the center electrode, at a position A which is shifted 0.1 mm from a forward end of an outer surface of the fusion portion toward the forward end side with respect to the direction of the axis, the width of the to-be-joined portion, measured on the cross section including the axis, is made smaller than the width of the gap-forming portion measured on the cross section, and the to-be-joined portion satisfies a relation Dtw/Dw ≤ 1.1, where Dtw represents the width (mm) of the to-be-joined portion at the position A measured on the cross section, and Dw represents the width (mm) of the fusion portion at the forward end of the outer surface thereof measured on the cross section.