A process for producing a methylol urea solution which comprises carrying out addition reaction between urea and formaldehyde in the presence of water and an alkali, wherein said alkali is a polyphosphoric acid salt; and a process for producing a slow-releasing nitrogenous fertilizer which comprises subjecting the resulting methylol urea solution to dehydration condensation and drying the resulting condensate are disclosed. The methylol urea solution undergoes no substantial pH variation with time either during or after the addition reaction, and the fertilizer obtained therefrom is excellent in slow-releasing properties.
