In an approach for testing the operations of a host system during a host system migration, a terminal agent exchanges messages already exchanged between the current host system and a terminal with the new host system. A manual operation replay unit replays messages generated by manual operations among the messages sent to the current host system by the terminal. An automatic response unit automatically generates a response message for messages received from the new host system. The automatic response unit also generates screen data for a screen displayed on the terminal on the basis of messages received from the new host system. A comparison unit compares and evaluates screen data generated by the automatic response unit and screen data from a screen generated by the terminal on the basis of messages received from the current host system.