A computer system in which multiple computer nodes are coupled via a communication network, wherein one computer node from among the multiple computer nodes becomes a leader computer node; other computer nodes besides the leader computer node from among the multiple computer nodes become follower computer nodes, each of the leader computer node and the follower computer nodes execute the same task, and each of the leader computer node and the follower computer nodes comprises: a communication interface part for communicating with other computer nodes via a computer network; a storage part for storing data; an access execution part for executing an access request for the storage part from a task; and an output determination part for determining whether or not a result of an execution executed by the access execution part is correct, and wherein the leader computer node: in the access execution part of its own node, creates a first output data on the basis of a first access request requested from a first task; and on the output determination part of its own node, queries the follower computer nodes as to whether or not the first output data is correct, and upon obtaining responses to the effect that the data is correct from a prescribed number of the follower computer nodes, determines that the first output data is correct, and wherein the follower computer nodes: in the access execution part of its own node, create a second output data on the basis of a second access request requested from a second task that performs the same processing as that of the first task; and in the output determination part of its own node, receive a query from the leader computer node, determines whether or not the first output data and the second output data are the same data, and in a case where the determination result is affirmative, send a response to the effect that the first output data is correct to the leader computer node. a communication interface part for communicating with other computer nodes via a computer network; a storage part for storing data; an access execution part for executing an access request for the storage part from a task; and an output determination part for determining whether or not a result of an execution executed by the access execution part is correct, and wherein the leader computer node: in the access execution part of its own node, creates a first output data on the basis of a first access request requested from a first task; and on the output determination part of its own node, queries the follower computer nodes as to whether or not the first output data is correct, and upon obtaining responses to the effect that the data is correct from a prescribed number of the follower computer nodes, determines that the first output data is correct, and wherein the follower computer nodes: in the access execution part of its own node, create a second output data on the basis of a second access request requested from a second task that performs the same processing as that of the first task; and in the output determination part of its own node, receive a query from the leader computer node, determines whether or not the first output data and the second output data are the same data, and in a case where the determination result is affirmative, send a response to the effect that the first output data is correct to the leader computer node. in the access execution part of its own node, creates a first output data on the basis of a first access request requested from a first task; and on the output determination part of its own node, queries the follower computer nodes as to whether or not the first output data is correct, and upon obtaining responses to the effect that the data is correct from a prescribed number of the follower computer nodes, determines that the first output data is correct, and wherein the follower computer nodes: in the access execution part of its own node, create a second output data on the basis of a second access request requested from a second task that performs the same processing as that of the first task; and in the output determination part of its own node, receive a query from the leader computer node, determines whether or not the first output data and the second output data are the same data, and in a case where the determination result is affirmative, send a response to the effect that the first output data is correct to the leader computer node. in the access execution part of its own node, create a second output data on the basis of a second access request requested from a second task that performs the same processing as that of the first task; and in the output determination part of its own node, receive a query from the leader computer node, determines whether or not the first output data and the second output data are the same data, and in a case where the determination result is affirmative, send a response to the effect that the first output data is correct to the leader computer node.