A gasifier start-up method for a gasifier capable of preventing or inhibiting a temporary occurrence of black smoke when a gasifier is started-up. A gasifier start-up method for a gasifier, in which a gasifying agent and a solid carbonaceous fuel are supplied and gasified, includes a start-up burner igniting step of supplying a start-up fuel and oxygen-containing gas to a start-up burner and igniting the start-up fuel and oxygen-containing gas, under an inert atmosphere in which an inside of the gasifier around the start-up burner is filled with inert gas; and an oxygen-containing-gas supply adjusting step of adjusting the supply of the oxygen-containing gas such that combustion gas obtained after the combustion reaction of the start-up fuel and the oxygen-containing gas becomes the inert gas, which contains little oxygen.