PURPOSE: To obtain an EC display element of a practical use region suitable for mass production by a simple method by forming a rhodium oxide layer by an ion plating method in an atmosphere contg. steam.

CONSTITUTION: After a transparent conductive film 2 is formed as a lower electrode on a transparent substrate 1, gaseous O2 is introducd to about 5×10-4 torr with a rhodium metal as a base material, after which H2O vapor is introduced to about 1×10-3 torr and a rhodium oxide layer 3 of about 2,000 film thickness is formed with substrate temp. layer forming speeds, etc. as parameters by an ion plating method (expressed hereunder as IP) in the presence of plasma. A large amt. of H2O is contained in said layer 3, but the absorption of the H2O varies with the vapor pressure of the introduced H2O and ion plating conditions and therefore the rhodium oxide layer contg. an arbitrary amt. of H2O in the layer is obtained by controlling both conditions.