We investigated to what extent humans are sensitive to snakes and spiders in a visual search task. In Experiment 1, fear-relevant deviants (snakes or spiders) were detected faster among fear-irrelevant backgrounds (flowers or mushrooms), than vice versa. Moreover, the detection of the fear-irrelevant target from snake backgrounds was significantly slower than that from spider backgrounds. It indicated that snakes held the attention more strongly than spiders did, that interfered with the effective visual search. In Experiment 2, fear-relevant animals (snakes or spiders) were compared with fear-irrelevant animals (birds or koalas). Fear-relevant animals were detected faster among fear-irrelevant animals, than vice versa. In addition, snakes were detected more rapidly than spiders, suggesting that snakes captured attention more strongly than spiders did. Furthermore, detection of the fear-irrelevant target (birds or koalas) from snake backgrounds was significantly slower than that from spider backgrounds. Again, it was indicated that snakes held the attention more strongly than spiders. We discussed our differences of the sensitivity to snakes and spiders in terms of evolutional origins of detecting hazardous animals.