A spark plug in which a glaze is applied to a rear trunk portion (245), a shoulder portion (240), and a portion of a intermediate diameter portion (230) of an insulator (200), and glaze firing is performed. Even when the glaze (shown by dots in the drawings) softened by heating flows downwards, the glaze is accommodated within a groove portion (235) formed between the shoulder portion (240) and the maximum diameter portion (210), and does not reach the maximum diameter portion (210). Such structure facilitates assembly of the insulator (200) to a metallic shell in a spark plug manufacturing process.