Synthesis of diamond by the so-called thin solvent film method is improved to obtain diamond crystals with a large grain size and good quality in an economical manner. To this end, using a reaction system comprising diamond seed crystal, a carbon source and a solvent metal, one or more masses of the solvent metal are arranged independently of each other and the diamond seed crystals are arranged in such a manner that one surface of the diamond seed crystal is contacted with each solvent metal mass.