It is well known that aging and longevity strongly correlate with energy metabolism. The nematode Caenorhabditis elegans is widely used as an ultimate model of experimental animals. Thus, we developed a novel tool, which is constructed from an optical detector, using an indirect method that can measure simply the energy metabolism of C. elegans. If we measure the oxygen consumption rate using this optical tool, we can easily evaluate the activity of mitochondria as an index in the aging process. However, a direct measurement of the oxygen consumption rate of C. elegans exposed in air is thought to be impossible because of the high concentration of atmospheric oxygen and the small size of the animals. We demonstrate here that we can directly detect the oxygen consumption with a small number of animals (<or=40) and a short accumulation time (<or=30 min) at high precision by using both an optical probe and a small chamber. The metabolic rate of a 4-day-old hermaphrodite animal, for example, was approximately 80 nW. Our method using C. elegans has the potential to become a useful technique for research on aging correlated with energy metabolism.