A spark plug 1 of the present invention includes an insulator 2 made from an alumina-based sintered body and including: a leg portion 22 provided on a front end side; a middle trunk portion 23 provided on a rear end side of the leg portion 22 and having a larger diameter than the leg portion 22; and a flange portion 24 provided on the rear end side of the middle trunk portion 23 and having a larger diameter than the middle trunk portion 23. In the spark plug 1, in a state where a cut surface 23b obtained by cutting the middle trunk portion 23 in a direction perpendicular to an axial line AX direction at an arbitrary position in the axial line AX direction is mirror-polished, when 20 observation regions X each being 185 μm×250 μm are set so as not to overlap each other, and with respect to each of a plurality of pores included in the 20 observation regions X, a square value P2 [μm2] of a length P [μm] of a circumference of the pore is obtained, an average value of the square values P2 [μm2] of top 20 pores having the largest square values P2 [μm2] is not greater than 2200 μm2, and a proportion T [%] of a total area of all pores included in the 20 observation regions X relative to a total area (100%) of the 20 observation regions X is not greater than 5%.