An alumina sintered body has a main phase made of alumina crystals, a first dispersed phase made of MgAl2O4 dispersed in the main phase, and a second dispersed phase made of SiO2 and/or CaO dispersed in the main phase. A content ratio of the main phase is within a range from not less than 98% to less than 98%. The alumina sintered body satisfies a relationship of 100×I2/I1≧5.5%, where I1 indicates a maximum peak height intensity derived from alumina, and I2 indicates a maximum peak height intensity derived from MgAl2O4 in a X-ray diffraction of the alumina sintered body. An area ratio of the second dispersed phase on a surface of the alumina sintered body is not more than 15.2%. A spark plug has a central electrode, a ground electrode and an insulator made of the alumina sintered body having the specific features previously described.