PURPOSE: To produce the above joined body by forming the single crystal of P type diamond as a uncleus and growing an N type cBN crystal on this single crystal.

CONSTITUTION: The raw material for BN is disposed in the high-temp. part in a growing vessel at the high temp. and high pressure stable for diamond and cBN, and this raw material is dissolved in a solvent contg. silicon. A temp. difference is provided in this solvent and the single crystal of the N type cBN is precipitated on the single crystal of the P type diamond disposed in the low-temp. part in the vessel by utilizing the difference in the rate of dissolution by temp. For example, a cylindrical heater 1 is used and the raw material 3 of cBN (powder of 325 to 400 meshes) and the lithium boronitride as the solvent 4 added with the powder silicon are packed into the Mo vessel 5. The single crystal 6 of the diamond having a P type semiconductor characteristic synthesized by a temp. difference method is disposed. The sample constituted in such a manner is set in an ultra-high pressure device and is treated for 40 hours at 55 kilobar pressure and 1450°C, by which the single crystal of the N type cBN is grown on the single crystal 6 as the uncleus.