Insulator and spark plug

A tubular insulator for a spark plug, wherein

• the insulator contains alumina as a primary component, and further contains a secondary component;
• the secondary component contains a silicon (Si) component, a magnesium (Mg) component, a barium (Ba) component, a calcium (Ca) component, and a rare earth component;
• and
• the amounts (mass %) of these components as reduced to oxide and relative to the total mass of the secondary component satisfy the following relations (1) to (5):
• 15.5≤(the amount of the Si component)≤55.1;
• (1)
• 0.6≤(the amount of the Mg component)≤7.4;
• (2)
• 26.7≤(the amount of the Ba component)≤68.5;
• (3)
• 1.1≤(the amount of the Ca component)≤32.4;
• (4)
• and
• 11.4≤(the amount of the rare earth component)≤46.5. (5)