A glass preform manufacturing method includes: generating glass fine particles by hydrolyzing a source gas in an oxyhydrogen flame; depositing the generated glass fine particles to form a torous glass preform; immersing the porous glass preform in an additive solution including an additive solvent in which a compound containing a desired additive is dissolved to impregnate the additive solution into the porous glass preform; first replacing of replacing the additive solvent remaining in the porous glass preform with the replacement solvent by immersing the porous glass preform in which the additive solution remains in a replacement solvent in which a solubility of the additive is lower than that in the additive solvent and having miscibility with the additive solvent; drying the porous glass preform after the first replacing; and sintering the dried porous glass preform to transparently vitrify the dried porous glass preform.