A granulation water control apparatus for a granulating machine used in a granular fertilizer production process and its granulation water control method. A particle size weight distribution of a semi-fabricated fertilizer discharged from the granulating machine is calculated, and a central particle size of the particle size weight distribution is calculated. The calculated central particle size and a previously set target central particle size are compared, and if they do not agree, a quantity of the granulation water to be charged into the granulating machine is calculated in accordance with a difference between the calculated central particle size and the target central particle size and the past statistical data.