A spark plug comprising: a center electrode; a ground electrode (8, 308, 408, 508, 608) disposed opposite to the center electrode with a gap therebetween; and a tip (9, 309, 409, 509, 609) joined to an opposed surface (31, 631) of the ground electrode (8, 308, 408, 508, 608) that is opposed to the center electrode, wherein the tip (9, 309, 409, 509, 609) has a discharge layer (40, 340, 440, 540, 640) and a relieving layer (50, 350, 450, 550, 650), the relieving layer (50, 350, 450, 550, 650) is formed from a Pt-Ni alloy and joined to the opposed surface (31, 631) via a diffusion layer, the diffusion layer has a gradient composition in which a Pt content increases and/or a Ni content decreases from the ground electrode (8, 308, 408, 508, 608) side toward the relieving layer (50, 350, 450, 550, 650) side, the discharge layer (40, 340, 440, 540, 640) is formed from a Pt-Rh alloy and joined via a clad diffusion layer to a side of the relieving layer (50, 350, 450, 550, 650) opposite to a side of the relieving layer (50, 350, 450, 550, 650) at which the ground electrode (8, 308, 408, 508, 608) is joined, the clad diffusion layer has a gradient composition in which a Pt content increases and/or a Ni content decreases from the relieving layer (50, 350, 450, 550, 650) side toward the discharge layer (40, 340, 440, 540, 640) side, and 0.81 ≤ A/B ≤ 1.21 is satisfied when an average cross-sectional area of a plurality of cross-sections of the discharge layer (40, 340, 440, 540, 640) obtained when the discharge layer (40, 340, 440, 540, 640) is cut along planes parallel to the opposed surface (31, 631) is A mm 2 and an average cross-sectional area of a plurality of cross-sections of the relieving layer (50, 350, 450, 550, 650) obtained when the relieving layer (50, 350, 450, 550, 650) is cut along planes parallel to the opposed surface (31, 631) is B mm 2 characterized in that C is greater than D where a thickness of the clad diffusion layer is C μm and a thickness of the diffusion layer is D μm. a center electrode; a ground electrode (8, 308, 408, 508, 608) disposed opposite to the center electrode with a gap therebetween; and a tip (9, 309, 409, 509, 609) joined to an opposed surface (31, 631) of the ground electrode (8, 308, 408, 508, 608) that is opposed to the center electrode, wherein the tip (9, 309, 409, 509, 609) has a discharge layer (40, 340, 440, 540, 640) and a relieving layer (50, 350, 450, 550, 650), the relieving layer (50, 350, 450, 550, 650) is formed from a Pt-Ni alloy and joined to the opposed surface (31, 631) via a diffusion layer, the diffusion layer has a gradient composition in which a Pt content increases and/or a Ni content decreases from the ground electrode (8, 308, 408, 508, 608) side toward the relieving layer (50, 350, 450, 550, 650) side, the discharge layer (40, 340, 440, 540, 640) is formed from a Pt-Rh alloy and joined via a clad diffusion layer to a side of the relieving layer (50, 350, 450, 550, 650) opposite to a side of the relieving layer (50, 350, 450, 550, 650) at which the ground electrode (8, 308, 408, 508, 608) is joined, the clad diffusion layer has a gradient composition in which a Pt content increases and/or a Ni content decreases from the relieving layer (50, 350, 450, 550, 650) side toward the discharge layer (40, 340, 440, 540, 640) side, and 0.81 ≤ A/B ≤ 1.21 is satisfied when an average cross-sectional area of a plurality of cross-sections of the discharge layer (40, 340, 440, 540, 640) obtained when the discharge layer (40, 340, 440, 540, 640) is cut along planes parallel to the opposed surface (31, 631) is A mm 2 and an average cross-sectional area of a plurality of cross-sections of the relieving layer (50, 350, 450, 550, 650) obtained when the relieving layer (50, 350, 450, 550, 650) is cut along planes parallel to the opposed surface (31, 631) is B mm 2 characterized in that C is greater than D where a thickness of the clad diffusion layer is C μm and a thickness of the diffusion layer is D μm.