It is well understood that sir2 (sirtuin), an NAD-dependent deacetylase, is essential for the extension of lifespan under caloric restriction. However, the mechanism underlying activation of sir2 is unclear. Life extension through caloric restriction requires the sir2 ortholog sir-2.1 in nematodes but occurs independently of the forkhead-type transcription factor DAF-16. We aimed here to elucidate the correlation between life extension in nematodes and NAD-dependent activation of sirtuin by analyzing the relationship between NAD and DAF-16. Lifespan was extended when Caenorhabditis elegans were bred using medium containing NAD. An RNA interference experiment revealed that life extension by NAD was sir-2.1 dependent. However, life extension by NAD did not occur in daf-16-RNAi nematodes, suggesting that NAD-dependent longevity requires daf-16. This result suggested that different signaling pathways are involved in life extension resulting from caloric restriction and from NAD addition. Expression of sod-3, a target gene of daf-16, and increased oxidative-stress resistance and adiposity were observed in response to NAD addition, indicating that NAD activated daf-16 in each phenotype. These results suggest that NAD affected lifespan through the activation of SIR-2.1 and DAF-16 along a signaling pathway, namely insulin-like signalling pathway (at least parts of it), different from that associated with caloric restriction.