A method for managing a tree structure comprising a migration process of positions of nodes on the tree structure in an information processing system in which a plurality of information processing devices are connected in the tree structure as nodes and messages are distributed according to the tree structure, the migration process comprising: causing a migrating node to form a connection with a parent node of a migration destination, which serves as a parent node of the migrating node after the migration; causing the migrating node to transmit a migration request to a root node in the tree structure; causing the root node to distribute a reply to the migration request according to the tree structure; causing a parent node of the migrating node before migration to stop distribution of a message (mB) to the migrating node after receiving the reply distributed according to the tree structure; causing the parent node of the migration destination, upon receiving the reply, to transmit the message (mB), which is received later than the reply, to the migrating node via the formed connection; causing the migrating node to store in a queue the message (mB) received from the parent node of the migration destination via the formed connection; causing the migrating node, upon receiving the reply distributed according to the tree structure, to reconstruct the distributed messages (mA, mB), based on a message (mA) which is received earlier than the reply received from the parent node before migration of the migrating node and the message (mB) stored in the queue which is received later than the reply; and causing the migrating node or the parent node before migration to disconnect the connection between the migrating node and the parent node before migration. causing a migrating node to form a connection with a parent node of a migration destination, which serves as a parent node of the migrating node after the migration; causing the migrating node to transmit a migration request to a root node in the tree structure; causing the root node to distribute a reply to the migration request according to the tree structure; causing a parent node of the migrating node before migration to stop distribution of a message (mB) to the migrating node after receiving the reply distributed according to the tree structure; causing the parent node of the migration destination, upon receiving the reply, to transmit the message (mB), which is received later than the reply, to the migrating node via the formed connection; causing the migrating node to store in a queue the message (mB) received from the parent node of the migration destination via the formed connection; causing the migrating node, upon receiving the reply distributed according to the tree structure, to reconstruct the distributed messages (mA, mB), based on a message (mA) which is received earlier than the reply received from the parent node before migration of the migrating node and the message (mB) stored in the queue which is received later than the reply; and causing the migrating node or the parent node before migration to disconnect the connection between the migrating node and the parent node before migration.