A spark plug 1 including an insulator 3 containing not less than 92 mass % and not greater than 96 mass % of Al component in terms of oxide, wherein the insulator is formed from an alumina sintered body comprising alumina crystal and a grain boundary phase present between crystal grains of the alumina crystal. Assuming that mass contents of an Si component, an Mg component, a Ba component, and a Ca component in terms of oxide are represented by M SiO2 , M MgO , M BaO , and M CaO , respectively, and a sum of M SiO2 , M MgO , M BaO , and M CaO is represented by Mt, the grain boundary phase contains these components so as to satisfy conditions (1) to (4) as follows: (1) 0.17 ‰¤ M SiO2 /Mt ‰¤ 0.47; (2) 0.005 ‰¤ M MgO /Mt ‰¤ 0.07; (3) 0.29 ‰¤ M BaO /Mt ‰¤ 0.77; (4) 0.03 ‰¤ M CaO /Mt ‰¤ 0.19.