A single-crystal diamond growth base material on which single-crystal diamond is grown having at least a base substrate of a material having a linear expansion coefficient smaller than that of MgO and not smaller than 0.5×10−6/K; a single-crystal MgO layer formed on a face of the base substrate where the single-crystal diamond is grown by a bonding method; and a film constituted of any one of an iridium film, a rhodium film, and a platinum film heteroepitaxially grown on the single-crystal MgO layer.