Start up method for hydrocarbon synthesis reaction apparatus

This start up method for a hydrocarbon synthesis reaction apparatus is provided with: an initial slurry-filling step in which, during start up, the inside of a reaction vessel is filled with an initial preparation slurry amount which is less than a slurry amount during steady operation; and a CO-conversion-rate increasing step in which hydrocarbons to be synthesized when operation is initiated are added to the slurry to increase the height of the liquid level of the slurry, and the CO conversion rate is increased in accordance with the increase in the height of the liquid level of the slurry.