To provide an alloy thin sheet for lead frames with an improved rust resistance by forming a uniform oxide film on its surface without adding trace amounts of special elements for promoting the formation of Fe oxide films.

In a process for manufacturing the alloy thin sheet for lead frames showing an excellent rust resistance, a hot-worked Fe-Ni alloy plate material comprising 38-50 mass% Ni, ≤0.1 mass% C, ≤1.0 mass% Si, ≤1.5 mass% Mn and the balance being Fe and unavoidable impurities is subjected, at least once each, to cold rolling and annealing in a reducing gas atmosphere, and subsequently to finish rolling and heating in an inert gas atmosphere at a temperature of >600°C to ≤750°C.