The objective of this paper is to find out a rule of cell optimization for fast contact detection in the discrete element method (DEM) algorithm. The contact detection process was executed by using the conventional cell model or distinct cell model (DCM) under several particle conditions having different particle size ratio and volume ratio, and its time was measured. The conventional cell model is suitable for the conditions that particle size ratio, d(large)/d(small), is small and its volume ratio, V-small/V-0, is also small. The optimum cell condition of the conventional cell model is that the mean number of particle in a cell is 1.0-5.0, regardless of the particle size ratio and its volume ratio. Contact detection using DCM, which has several grids for different sized particles, is much faster than that of the conventional model when particles have a large size ratio. The size of the larger cell does not affect the contact detection speed; however, the size of the smaller cell is important for the fast contact detection and its size is determined to be about 3.0 times the particle radius. Therefore, the optimum cell condition is estimated from the particle size ratio and its volume ratio, and a large-scale DEM will be possible.