A method of preparing a sulfur-containing compound having at least one thio group in the molecules by reacting a thiol compound with an organic compound having at least one functional group capable of forming a thio group in the molecules upon reacting with a mercapto group in the presence of a basic compound, wherein the number of moles of water contained in the reaction system is set to be not larger than 7.5 times as large as the number of moles obtained by multiplying the number of moles of the thiol compound to be reacted by a number of mercapto groups present in one molecule of the thiol compound.