A ceramic sintered body having a variety of excellent characteristics, such as strength, heat resistance, corrosion resistance, wear resistance and lightweight and the like, as compared with the conventional metal materials has been widely used in a mechanical part, functional part, structural material and decorative material for forming a semiconductor board, electronic equipment material, engine part, material for a high-speed cutting tool, nozzle, bearing or the like that is used in severe temperature, stress and wear conditions under which conventional metal materials cannot be used satisfactorily. 
Particularly, since an aluminum nitride (AlN) sintered body is an insulating body having excellent thermal conductivity and a thermal expansion coefficient near that of silicon (Si), it has been further widely used as a heat radiation plate or a substrate of a highly-integrated semiconductor apparatus. 
The above conventional aluminum nitride sintered body has been usually mass-produced in accordance with the following manufacturing method.  That is, a sintering agent such as Y2O3 or the like, an organic binder and, if necessary, any of various additives, a solvent and a dispersant are added to a raw material powder of aluminum nitride thereby to prepare a material powder mixture.  The obtained raw material powder mixture is molded into a thin-plate shape or sheet-shape molded body by a doctor blade method or a slip casting method, or the raw material powder mixture is press-molded into a thick-plate or a large-size molded body.  Then, the thus obtained molded body is heated to a temperature of 400-500° C. in the air or nitrogen atmosphere, and is dewaxed so that carbon hydride component and the like used as the organic binder is removed from the molded body and is almost completely dewaxed.  The dewaxed molded body is heated to high temperature in nitrogen atmosphere or the like so as to be densified and sintered so that an aluminum nitride sintered body is formed. 
Aluminum nitride is a sinter-resisting ceramic.  In order to promote a densification of the ceramic and to prevent the thermal resistance of the ceramic from increasing by solid-dissolving impurity oxygen contained in AlN material powder into the AlN crystal grains, there has been generally adopted a method in which rare earth oxide such as yttrium oxide (Y2O3) or the like is used as the sintering agent.  These sintering agents react with oxygen contained in AlN material powder.  In case of Y2O3, there is formed a liquid phase composition composed of 3Y2O3.5Al2O3 (YAG), Y2O3.Al2O3 (YAL), 2Y2O3.Al2O3 (YAM) or the like, thereby to achieve the densification of the sintered body.  In addition, the impurity oxygen acting to increase the thermal conductivity can be fixed as grain boundary phase, so that the high thermal conductivity of the sintered body can be achieved. 
As an example of such conventional high thermal aluminum nitride sintered body, Japanese Patent Application (Laid Open) No.10-25160 has proposed an aluminum nitride sintered body comprising: main phase composed of aluminum nitride of which an average grain size of aluminum nitride crystal grains is 2-10 μm; and secondary phase composed of a single component of 2Y2O3.Al2O3, Y2O3.Al2O3 or 3Y2O3.5Al2O3 content of 1.0-4.6 wt %; wherein the aluminum nitride sintered body has a thermal conductivity of 200 W/mK or more, and a bending strength of 40 Kg/mm2 or more. 
In the above conventional manufacturing method, however, even in a case where an average grain size of the material powder, kinds and addition amounts of impurity and sintering assistant agent, conditions of the degreasing treatment and sintering operation or the like were strictly controlled, it was difficult to obtain a AlN sintered body having a high thermal conductivity of 220 W/m·K or more.  Namely, since a large amount of rare earth oxide to be 3-5 mass. %  required for fixing the aforementioned impurity oxygen is added to the AlN material powder, an amount of oxide acting as thermal resistance is increased in a structure of the sintered body, so that it is difficult to obtain a AlN sintered body having a high thermal conductivity of 220 W/m·K or more.  Accordingly, there are many cases where an excellent heat radiating property which is the most characterizing property inherent to AlN sintered body is disadvantageously impaired, so that a technical improvement has been eagerly demanded. 
The present invention has been achieved for solving the foregoing problems and therefore an object of the present invention is to provide a high thermal conductive aluminum nitride sintered body having a high thermal conductivity and is excellent in heat radiating property.