A heat insulator including a porous sintered body having a porosity of 70 vol % or more, pores having a pore size of more than 1000 μm in a proportion of 10 vol % or less of all pores and pores having a pore size of 0.8 μm or more and less than 10 μm occupy 50 vol % or more and 80 vol % or less of pores having a pore size of 1000 μm or less, while pores having a pore size of 0.01 μm or more and less than 0.8 μoccupy 10 vol % or more and 30 vol % or less pores having a pore size of 1000 μm or less. The porous sintered body is formed from MgAl2O4 raw material and includes a fibrous layer formed from inorganic material fibers, the heat conductivity of the heat insulator at 1000° C. or more and 1500° C. or less being 0.40 W/m·K) or less.