In order to assess the protective properties of rust films formed on weathering steel, electrochemical AC impedance and harmonic current measurements were made in 0.1 M Na₂SO₄ solution, and an equivalent circuit for rusted weathering steel was obtained. Through analysis of the equivalent circuit, the protective properties of rust films were found to be dominated by ion migration resistance through the rust films. Based on these results, a portable instrument was developed for nondestructive and quantitative evaluation of the protective properties of rust films on weathering steel. Many existing structures and test pieces made of weathering steel exposed throughout Japan were inspected by this instrument, and a database of rust conditions were accumulated. Through the analysis of the database, an empirical criterion for the patina condition was clarified. The application of this instrument to a field structure is also reported.