Disclosed is a spark plug having an insulator capable of achieving good breakage resistance by modifying the state of contact of the insulator with a plate packing and the like without increase in insulator thickness. A spark plug 1 contains a ceramic insulator 2, a plate packing 22 and a metal shell 3. The ceramic insulator 2 has, on an outer circumferential surface thereof, a step portion 14 having an outer diameter decreasing toward the front in the direction of an axis CL1 and a leg portion 13 extending toward the front in the direction of the axis CL1. The metal shell 3 has, on an inner circumferential surface thereof, a taper portion 21 having an inner diameter decreasing toward the front in the direction of the axis CL1. The ceramic insulator 2 is fixed in the metal shell 3 with the step portion 14 retained on the taper portion 21 via the plate packing 22. The ceramic insulator 2 also has a curved surface portion 31 formed into a concave shape on the outer circumferential surface thereof at a position between the step portion 14 and the leg portion 13. Herein, 50% or more of an inner circumferential edge portion IP of the plate packing 22 in a circumferential direction thereof is in contact with a part of the ceramic insulator 2 located front of a middle region CP of the curved surface portion 31.