A mullite sintered body according to the present invention has an impurity element content of 1% by mass or less and contains sintered mullite grains having an average grain size of 8 μm or less. When a surface of the mullite sintered body is finished by polishing, pores in the surface have an average largest pore length of 0.4 μm or less. The surface preferably has a center line average surface roughness (Ra) of 3 nm or less. The surface preferably has a maximum peak height (Rp) of 30 nm or less. The number of pores in the surface is preferably 10 or less per unit area of 4 μm×4 μm.