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The number of traffic deaths in Japan was 4,914 in 2009. Since the head was the most common site of injury in traffic accidents (2,302, 47%), traumatic brain injury causes the fatalities in these accidents. The aim of the present study was to quantify micro injuries in the animal brain for gaining insight and understanding of the human brain injury tolerance. Using porcine brain matter, in vitro stress relaxation experiments and in vivo impact experiments were conducted. In both experiments, the distribution of the damage ratio of the transverse to longitudinal length of cells, hereafter, referred to as an aspect ratio, in the brain matter under loading was examined. In the in vitro stress relaxation experiments, specimens were compressed vertically with a compression velocity of 1 mm/s, and the displacement was held for 140 sec when the compression strain reached the target strain. In the experiments, there were five categories of compression strain: 10, 20, 30, 40, and 50 percent.