To provide a production process with high degree of conversion of HFC-134a and selectivity for HFO-1123, with a high productivity of HFO-1123 and with a small load in process of purification and recovery.A process for producing HFO-1123, which comprises bringing a material gas having a proportion of HFC-134a based on the total amount of a diluent gas and HFC-134a of from 50 to 100 mol % into contact with a dehydrofluorination catalyst to convert part of HFC-134a into HFO-1123, then removing hydrogen fluoride in a reaction product gas, and then bringing the reaction product gas from which hydrogen fluoride has been removed into contact with a dehydrofluorination catalyst to convert at least part of unreacted HFC-134a into HFO-1123.