PROBLEM TO BE SOLVED: To provide a birth rate increase promoter applied to mammals, which can increase provability of impregnation and delivery of mammals including livestock, pets, etc.SOLUTION: The birth rate increase promoter contains an isoprenoid compound. More specifically, the isoprenoid compound is at least one substance selected from geranylgeranoic acid, farnesoic acid, geranylgeranylamine, farnesylamine, geranylamine, geranylgeraniol, geranylgeranial, geranylgeranyl pyrophosphoric acid and 4,5-didehydro geranylgeranoic acid.