A start-up method for a hydrocarbon synthesis reaction apparatus, comprising: an initial slurry-loading step in which the slurry is loaded into the reactor at the initial stage of the Fischer-Tropsch synthesis reaction at a lower loading rate than that applied to the reactor in a steady-state operation; and a CO conversion ratio-increasing step in which the liquid level of the slurry in the reactor is raised by adding to the slurry the hydrocarbons synthesized at the early stage of the Fischer-Tropsch synthesis reaction so that the CO conversion ratio is increased in proportion to a rise in the liquid level of the slurry in the reactor.