An optical fiber preform manufacturing method comprising: a first drying step of drying two or more kinds of alkali metal salt raw materials containing mutually different alkali metal elements at a temperature equal to or lower than a lowest temperature among melting point temperatures of the two or more kinds of alkali metal salt raw materials; a second drying step, after the first drying step, of drying the two or more kinds of alkali metal salt raw materials at a temperature which is equal to or higher than a highest temperature among the melting point temperatures of the two or more kinds of alkali metal salt raw materials and at which vapor pressures of all the two or more kinds of alkali metal salt raw materials are 266.645 Pa (2 mmHg) or less; a thermal diffusion step, after the second drying step, of thermally diffusing two or more kinds of alkali metal element oxides on an inner surface of a silica-based glass pipe, the thermal diffusion step including simultaneously heating the two or more kinds of alkali metal salt raw materials to generate vapor of the two or more kinds of alkali metal salt raw materials, and supplying the vapor and a carrier gas from one end side of the glass pipe to an inside of the glass pipe while heating the glass pipe with a heat source to cause an oxidation reaction of two or more kinds of alkali metal elements contained in the vapor, the heat source relatively moving in a longitudinal direction of the glass pipe; a core rod fabrication step, after the thermal diffusion step, of collapsing the glass pipe to fabricate a core rod; and a cladding portion addition step, after the core rod fabrication step, of adding a cladding portion on an outer periphery of the core rod, wherein a difference among the melting point temperatures of the two or more kinds of alkali metal salt raw materials is 50° C or lower. a first drying step of drying two or more kinds of alkali metal salt raw materials containing mutually different alkali metal elements at a temperature equal to or lower than a lowest temperature among melting point temperatures of the two or more kinds of alkali metal salt raw materials; a second drying step, after the first drying step, of drying the two or more kinds of alkali metal salt raw materials at a temperature which is equal to or higher than a highest temperature among the melting point temperatures of the two or more kinds of alkali metal salt raw materials and at which vapor pressures of all the two or more kinds of alkali metal salt raw materials are 266.645 Pa (2 mmHg) or less; a thermal diffusion step, after the second drying step, of thermally diffusing two or more kinds of alkali metal element oxides on an inner surface of a silica-based glass pipe, the thermal diffusion step including simultaneously heating the two or more kinds of alkali metal salt raw materials to generate vapor of the two or more kinds of alkali metal salt raw materials, and supplying the vapor and a carrier gas from one end side of the glass pipe to an inside of the glass pipe while heating the glass pipe with a heat source to cause an oxidation reaction of two or more kinds of alkali metal elements contained in the vapor, the heat source relatively moving in a longitudinal direction of the glass pipe; a core rod fabrication step, after the thermal diffusion step, of collapsing the glass pipe to fabricate a core rod; and a cladding portion addition step, after the core rod fabrication step, of adding a cladding portion on an outer periphery of the core rod, wherein a difference among the melting point temperatures of the two or more kinds of alkali metal salt raw materials is 50° C or lower. <EOS>