In this paper, we propose an algorithm for state estimation and attack detection in distributed observer system and derive the condition for achieving the estimation and detection. In detail, the algorithm using the notion of virtual states can be used in each local observer for distributedly attack detection by oneself. In the perspective of attack, we define, not only measurement attack, but an observer communication attack which is the falsification of the transmitted data between local observers and also define a combination attack which combines the observer communication attack and measurement one. We derive the condition for estimation and detection under the combination attack as the theorem and, finally, verify the effectiveness of the proposed algorithm by numerical simulations.