PURPOSE: To manufacture a heat collecting body that exccels in the selective absorbing characteristics of the solar heat in a simple and economical process by forming a selective absorbing film on the surface of a base material having iron as a primary constituent by the electrolytic oxidizing method.

CONSTITUTION: An iron oxide having triiron tetroxide as a primary constituent, or a composite material of such iron oxide and a metallic element (oxide) constituting the electrolyte is formed on the surface of a base material having iron as a primary constituent to be used as a selective absorbing film of a solar heat collecting body. For the electrolyte to perform the electrolytic oxidizing, a pure or blended liquid of an acid oxidizing agent (for instance, bichromate such as potassium bichromate or permanganate etc.) having a metallic complex ion as anion and a metal as cation that has an ionization tendency greater than iron is most suitable. Or, a pure or blended liquid of an acid oxidizing agent (for instance, nitrate, nitrite, peroxide salt etc.) having metal that shows an ionization tendency geater than iron as cation may also be used.