In this paper, we propose virtual distributed observers which are built by virtually excluding some redundant sensors from conventional distributed observers. By using these observers, we can construct not only robust estimators against sensor attacks but detectors of attacks by malicious agents. We also consider the new framework of estimation problem which is more useful to adapt under distributed conditions. This relaxed problem makes it possible to estimate partial state under sensor attacks even if the whole observer system is not detectable. Finally, we confirm the effectiveness of our proposed method through numerical simulations.