Spark plug that has an insulator less likely to be damaged

A spark plug comprising an insulator in which an axial hole extending along an axial line from a front side to a rear side is formed, wherein

• the insulator includes
• a cylindrical first portion,
• a cylindrical second portion located on the rear side of the first portion, an outer diameter F at a front end of the second portion being smaller than an outer diameter of the first portion,
• a cylindrical third portion located on the front side of the first portion, an outer diameter D at a rear end of the third portion being smaller than the outer diameter of the first portion and the outer diameter F at the front end of the second portion,
• a first slope portion connecting the first portion and the second portion, and having an outer diameter that reduces toward the rear side, and
• a second slope portion connecting the first portion and the third portion, and having an outer diameter that reduces toward the front side, and
• a length A in an axial-line direction of the second slope portion and a length B in the axial-line direction of the first slope portion satisfy 2.0≤A/B≤3.9, and 0.50≤D/F≤0.88 is satisfied.