To prevent high temp. oxidation of an aluminum alloy by constituting an adhering liquid consisting of a substance containing fluorine, which generates a gas containing fluorine at a heat treatment temp. and a binder agent, which is adhered on an aluminum alloy surface and to form a liquid phase, and coating an aluminum alloy surface.

A substance containing fluorine genrates at 529°C a fluorine containing gas of a partial pressure of 0.1-3 bar. Fluoric borate is preferably form the substance containing fluorine. A binder agent contains three compound A, B, C, consisting of an alkali metallic salt (A) of an organic mono-carboxylic fatty acid having a carbon number of 10-22, an alkali earth metallic salt (B) of an organic mono-carboxylic fatty acid having a carbon number of 10-22 and a compound containing boron (C) selected from borate and condensation borate. A adhering liquid is constituted so that a ratio between a substance containing fluorine and the binder agents (A+B) is 0.0008:1 to 0.1:1. A solid coating layer is formed by coating the adhering liquid on an aluminum alloy and drying.