An alumina sintered body of the present invention has alumina crystals as a main phase and an amorphous grain boundary glass phase. The amorphous grain boundary glass phase is a grain-boundary glass phase having an amorphous glass component in which at least one of either CaO or MgO is added to SiO2, and at least one type of oxide selected from rare-earth elements and elements in Group IV of the periodic table included in the amorphous glass component as a specific component. When a composition ratio of the main phase and the amorphous grain boundary glass phase is alumina:amorphous glass component:specific component=a:b:c (a+b+c=100% by weight), in a triangular coordinate of which peaks are alumina, the amorphous glass component, and the specific component, a point (a,b,c) is within a range surrounded by four points, A(98.0,1.0,1.0), B(90.0,5.0,5.0), C(93.5,5.0,1.5), and D(97.8,2.0,0.2).