Perfluorodiacyl fluorides compounds are important precursors for production of monomeric starting materials for heat-resistant and chemical-resistant fluororesins.  For example, CF2═CF—O(CF2)3COOCH3, CF2═CF—OCF2CF(CF3)O(CF3)3COOCH3, CF2═CF—O(CF2)3CH2COOCH3 and the like are known as perfluoro(alkyl vinyl ethers) having a carboxyl group in the molecule useful as starting monomers for ion exchange membranes (JP-A-52-153897). 
These perfluoro(alkyl vinyl ethers) are produced via perfluoro diacyl fluorides (J. Fluorine Chem.,  94, 65-68 (1999)).  In particular, the starting monomer for mechanically strong ion exchange membranes, CF2═CF—O(CF2)3COOCH3, is derived from FCO(CF2)2O(CF2)2COF or FCOCF(CF3)O(CF2)2COF. 
It is known that diacyl fluorides are generally produced by the following method using iodine and fuming sulfuric acid. CF2═CF2+I2→ICF2CF2IICF2CF2I+CF2═CF2→ICF2CF2CF2CF2IICF2CF2CF2CF2I+SO3→FCOCF2CF2COF
The present inventors proposed a process for producing perfluoro(diacyl fluorides) from diols by fluorination with fluorine in a liquid phase (liquid phase fluorination) (WO02/4397). 
On the other hand, for fluorination of C—H into C—F, fluorination with elemental fluorine is known. 
For production of compounds having fluorinated vinyl groups at both ends, it is proposed to treat a compound having CF2═CF— at one end and having —COF at the other end with chlorine gas or the like to add halogen atoms to CF2═CF—, thermally decompose the terminal —COF into CF2═CF— and then restore CF2═CF—by dehalogenation (JP-A-1-143843). 
However, conventional processes for producing perfluorodiacyl fluoride compounds from tetrafluoroethylene have a problem of the economical disadvantage of the expensive starting material.  Further, there are problems of corrosion of the equipment resulting from use of iodine and fuming sulfuric acid and difficulty in handling the reaction reagents.