The present inventors have found that an aluminum nitride sintered body excellent in flatness by polishing can be provided by improving the strength of a grain boundary; at the same time, by adding SiO2 or MgO, which forms a solid solution with aluminum nitride during a sintering process, with the result that it is no longer present as a grain boundary phase, in a small amount to an aluminum nitride powder, followed by sintering the aluminum nitride powder at a low temperature from more than 160° C. to less than 1750° C.