PURPOSE: To remarkably facilitate the control of growth velocity of diamond in case of producing diamond crystal by a two-stage pressurization operating method by previously interposing metallic flux between carbon source substance and diamond crystal for a seed.

CONSTITUTION: A magnesia plate 4, diamond crystal 1 for a seed, metallic flux 2 and carbon source substance 3 are introduced into a vessel with this order and the vessel is set to a high-pressure and high-temp. apparatus. While holding the vessel at the temp. not less than the action temp. of both metallic flux 2 and carbon source substance 3 at the pressure not exceeding the lowermost pressure capable of forming a spontaneous neuclus for diamond from carbon source substance 3, a first stage adding pretreatment to carbon source substance 3 is performed. Thereafter such as second stage that the pressure is ascended and kept in a range exceeding the equibrium pressure of graphite and diamond and not exceeding the lowermost pressure capable of forming the spontaneous nucleus for diamond from carbon source substance subjected to the pretreatment is performed. Thereby the control of growth velocity of diamond crystal is remarkably facilitated by the arrangement of metallic flux.