Spark plug insulator and spark plug

A spark plug insulator composed of an alumina-based sintered body provided with an axial hole extending along an axial line, wherein

• when a force to bend the spark plug insulator is applied to the spark plug insulator to fracture the spark plug insulator,
• for a range including a starting point of the fracture out of two ranges into which a fracture surface created by the fracture is divided by a plane which is a plane perpendicular to a direction of the force and includes the axial line,
• an average of areas of particles appearing in a planar image of the range is 4.4 μm2 or larger and 8.0 μm2 or smaller, and a maximum area of the particles is 600 μm2 or smaller,
• the particles include large particles having an area of 60 μm2 or larger and 600 μm2 or smaller, and
• as the large particles, 0.1 particles/mm2 or more are present per unit area of the planar image.