PROBLEM TO BE SOLVED: To provide a gas-phase polymerization process for preparing polyolefins which process has no inconvenience for a high apparatus cost and avoids substantially the deposit of polymers in a gas-phase reactor, while keeping the public-known advantageous performance of a gas-phase polymerization process by the indirect cooling or partial condensation of a circulating gas. SOLUTION: The continuous gas-phase polymerization process for preparing polyolefins, carrying out a polymerization while removing a reaction heat by circulating a fluid gas in a fluidized bed reactor, is characterized in that a fluid gas discharged from the fluidized bed reactor is cooled directly by means of a cooling apparatus, and a coolant heated by the direct contact with the fluid gas is again cooled, then the coolant after the cooling is directly returned to the cooling apparatus, in the mean time, the coolant heated or cooled is returned to the fluidized bed reactor.