To provide an economically advantageous process for producing industrially useful HFO-1234yf efficiently and in a sufficiently controlled state by one reaction involving thermal decomposition, by using readily available raw material. A process for producing 2,3,3,3-tetrafluoropropene from a raw material composition containing chlorodifluoromethane and chloromethane, by a synthetic reaction involving thermal decomposition, which comprises (a) a step of supplying the chlorodifluoromethane and the chloromethane to a reactor, as preliminarily mixed or separately, in such amounts that the chloromethane would be in a ratio of from 0.01 to 3 mol to 1 mol of the chlorodifluoromethane, (b) a step of supplying a heat medium to the reactor, and (c) a step of bringing the heat medium in contact with the chlorodifluoromethane and the chloromethane in the reactor to form the 2,3,3,3-tetrafluoropropene.