Hydrothermal reactions of formaldehyde (CH2O), which is mainly formed by CO2, are considered to be of most importance in the abiotic synthesis of complex organic molecules in chemical evolution on the primitive Earth. We show the hydrothermal formation of amino acids and oligopeptides in neutral or weak acidic solutions from CH2O in the presence of metals and metal oxides. In these hydrothermal processes, metal or metal oxides selectively catalyzed a series of hydrothermal reactions from CH2O to amino acids, whereas the further reactions for the formation of oligopeptides proceeded without any catalyst. Our experimental observation may provide a possible clue to understanding the initial chemical evolution in Earth's primitive atmosphere of the origin of life.