  We investigate applicability of GPU to DEM. NVIDIA's code obtained superior
performance than CPU in computational time. A model of contact forces in
NVIDIA's code is too simple for practical use. We modify this model by
replacing it with the practical model. The simulation shows that the practical
model obtains the computing speed 6 times faster than the practical one on CPU
while 7 times slower than the simple one on GPU. The result are analyzed.
