A method for producing ditrimethylolpropane (di-TMP) by recovering the di-TMP from a reaction solution containing trimethylolpropane, the di-TMP, tritrimethylolpropane (tri-TMP) and bistrimethylolpropane (bis-TMP), which is produced by reacting n-butyl aldehyde and formaldehyde in the presence of a base catalyst, said method comprising the steps of: reacting n-butyl aldehyde and formaldehyde in the presence of the base catalyst to obtain the reaction solution containing the TMP, the di-TMP, and the bis-TMP and/or the tri-TMP as by-products; distilling the reaction solution to remove unreacted formaldehyde therefrom and obtain the TMP-containing solution; subjecting the TMP-containing solution to extraction with the organic solvent to separate a reaction product-containing extract therefrom; distilling the extract obtained in the step to remove the organic solvent therefrom and obtain the crude TMP; distilling the crude TMP to separate the purified TMP therefrom and obtain a di-TMP-containing solution from the distillation still residue; further distilling the di-TMP-containing solution to separate a di-TMP-containing solution as a distillate therefrom; and subjecting the di-TMP-containing solution to crystallization with the organic solvent containing an aliphatic ketone to obtain di-TMP, said method being carried out under the following conditions and: the di-TMP-containing solution obtained in the step contains the bis-TMP and the tri-TMP in amounts of 10 parts by mass or less and 5 parts by mass or less, respectively, on the basis of 100 parts by mass of the di-TMP; and in the step, the crystallization is initiated under pressure at a temperature exceeding a boiling point of the organic solvent as measured under normal pressures, and the resulting crystallization product solution is cooled at a temperature drop rate of 2°C/min or less. reacting n-butyl aldehyde and formaldehyde in the presence of the base catalyst to obtain the reaction solution containing the TMP, the di-TMP, and the bis-TMP and/or the tri-TMP as by-products; distilling the reaction solution to remove unreacted formaldehyde therefrom and obtain the TMP-containing solution; subjecting the TMP-containing solution to extraction with the organic solvent to separate a reaction product-containing extract therefrom; distilling the extract obtained in the step to remove the organic solvent therefrom and obtain the crude TMP; distilling the crude TMP to separate the purified TMP therefrom and obtain a di-TMP-containing solution from the distillation still residue; further distilling the di-TMP-containing solution to separate a di-TMP-containing solution as a distillate therefrom; and subjecting the di-TMP-containing solution to crystallization with the organic solvent containing an aliphatic ketone to obtain di-TMP, the di-TMP-containing solution obtained in the step contains the bis-TMP and the tri-TMP in amounts of 10 parts by mass or less and 5 parts by mass or less, respectively, on the basis of 100 parts by mass of the di-TMP; and in the step, the crystallization is initiated under pressure at a temperature exceeding a boiling point of the organic solvent as measured under normal pressures, and the resulting crystallization product solution is cooled at a temperature drop rate of 2°C/min or less.