the present invention is explained in detail below with reference to the examples . a 10 % aqueous ammonia ( 114 g ) was added to a 5 . 7 % chromium nitrate solution ( 765 g ), obtaining a precipitate . the precipitate was filtered , washed , and dried in air at 120 ° c . for hours , producing chromium hydroxide . the thus - obtained chromium hydroxide was formed into pellets having the size of 3 . 0 mm in diameter and about 3 . 0 mm in height . the pellets were then calcined in a nitrogen stream at 400 ° c . for 2 hours . according to the results of the quantitative analysis of cr and elemental analysis , the resulting chromium oxide was confirmed to have a composition formula of cro 2 . 0 . the pelleted chromium oxide was placed in a reaction tube made of hastelloy c ; hydrogen fluoride was diluted to 20 vol % using nitrogen and supplied to the reaction tube ; the tube was heated by increasing the temperature step by step from 200 to 360 ° c . ; and fluorination was then conducted using 100 % hf for 220 hours . the resulting fluorinated chromium oxide had a specific surface area ( measured by the bet method ) of 70 m 2 / g and the fluorine content was 31 . 4 weight . according to the step schematically shown in fig1 , 2 , 3 , 3 , 3 - tetrafluoropropene ( hfc - 1234yf ) was synthesized in the process described below . first , 2 kg of highly fluorinated - chromium oxide catalyst prepared in the process described above was placed in a reaction tube made of hastelloy c , and the temperature of the catalytic layer was maintained at 400 ° c . vaporized 1 , 1 , 1 , 2 , 3 - pentafluoropropane ( hfc - 245eb ) was supplied to this reaction tube at 3 l / min ( the flow rate at 0 ° c . and 0 . 1 mpa ) ( stream f11 ). the starting material carried 3 mol % oxygen . the contact time w / fo at this time was 40 g · sec / cc . note that f 0 represents the flow rate of hfc - 245eb at 0 ° c . and 0 . 1 mpa . the outflow gas ( stream s11 ) from the reaction tube was cooled , and placed in a rectification column having a size of 2 m in height and 7 cm in diameter , a mixture of the objective hfo - 1234yf and hf was obtained at the top of the column ( stream s12 ). from the bottom of the column , unreacted hfc - 245eb and hfo - 1234ze and hfc - 245fa as by - products were extracted ( stream s13 ). the mixture ( stream s13 ) extracted from the bottom of the column was placed in an hf collecting apparatus and washed with water to remove hf , dried using a molecular sieve , re - supplied to the reaction tube ( stream s14 ), and contacted with a highly fluorinated - chromium oxide catalyst along with the hfc - 245eb as a starting material under the conditions described above . table 1 shows the flow rate ( kg / hr ) of each component of each stream . because hfc - 245eb ( i . e ., starting material ) was not detected in the outflow gas from the reaction tube , it can be concluded that the conversion rate of hfc - 245eb was substantially 100 %. as is clear from table 1 , the flow rate of hfo - 1234ze and hfc - 245fa ( s14 ) that were re - supplied to the reaction tube was the same as that of the hfo - 1234ze and hfc - 245fa in the outflow gas ( s11 ) from the reaction tube . in other words , in spite of the continuous reaction , the flow rate of hfo - 1234ze and hfc - 245fa was maintained at a constant level without increase . from these results , it was confirmed that the by - product from the dehydrofluorination was converted into hfo - 1234yf by being re - supplied to the reaction tube . if not recycled , hfo - 1234ze and hfc245fa will cause a large loss . by re - supplying hfo - 1234ze and hfc245fa ( i . e ., by - products ) to the reaction tube , in which the dehydrofluorination of hfc - 245eb is conducted , to use the by - products in the dehydrofluorination reaction and isomerization reaction , the equipment cost and loss of hfo - 1234ze and hfc245fa can be reduced . dehydrofluorination of 1 , 1 , 1 , 2 , 3 - pentafluoropropane ( hfc - 245eb ) and recycling of the by - products were conducted in the same manner as in example 1 except that hfc - 245eb was supplied at 4 l / min ( the flow rate at 0 ° c . and 0 . 1 mpa , w / fo = 30 g · sec / cc ). in this case , the conversion rate of hfc - 245eb was substantially 100 %. table 2 shows the flow rate ( kg / hr ) of each component of each stream . dehydrofluorination of hfc - 245eb and recycling of the by - products were conducted in the same manner as in example 1 except that the reaction temperature was set at 450 ° c . in this case , the conversion rate of hfc - 245eb was substantially 100 %. table 3 shows the flow rate ( kg / hr ) of each component of each stream .