A nanocomposite metal material includes a carrier formed of Zr and two-element metal particles supported on the carrier. The two-element metal is formed of Cu and Ni, and a degree of oxidation of the carrier is more than 31% and 100% or less. In a case where the nanocomposite metal material is disposed in a reaction furnace of a thermal reactor, the inside of the reaction furnace is brought into a vacuum state, and the inside of the reaction furnace is heated to a temperature range of 250° C. or higher and 350° C. or lower with a heating mechanism included in the thermal reactor while supplying at least one of hydrogen gas and deuterium gas into the reaction furnace, excessive heat of the nanocomposite metal material is 100 W/kg or more.