A process for producing propylene efficiently from a raw material mixture comprising an olefin having four or more carbon atoms and at least one of methanol and dimethyl ether with high selectivity and in a high yield by inhibiting the production of by-products such as aromatic compounds or paraffins while keeping the reaction rate at a high level. The process comprises diluting a raw material mixture comprising an olefin having four or more carbon atoms and at least one of methanol and dimethyl ether with a diluent (C) (which consists of one or more members selected from among paraffins, aromatic compounds, steam, carbon dioxide, carbon monoxide, nitrogen, argon, and helium)in such a way as to satisfy the requirement (A) that the ratio of the amount by mole of the olefin fed into the reactor to the sum of the amount by mole of methanol fed into the reactor and twice the amount by mole of dimethyl ether fed thereinto is 0.5 to 10 and the requirement (B) that the total concentration of the olefin, methanol and dimethyl ether in the whole component fed into the reactor is 20 to 80% by volume, and subjecting the obtained dilution to catalytic reaction.