Abstract: A fluidized bed or fluidized bed/spouted bed granulator into which urea seed particles, an aqueous urea solution and air are introduced to produce urea particles with an average particle size of 1 mm or more. The granulator includes a box-shaped granulation chamber with a bottom floor, a top and side surfaces. Inner wall surfaces of the chamber are of a metal plate material, an upper part of the side or top surfaces includes an exhaust outlet, the side surfaces or top surface includes an inlet for introducing the seed particles, the side surfaces include a collection port for the particles, the aqueous urea solution is introduced from the bottom floor or the side surfaces, the air is introduced from the bottom floor or the bottom floor and the side surfaces, and at least a portion of the inner wall surfaces of the granulation chamber is treated by surface roughening.