In order to provide a user with an application customized according to a situation of a task, there is provided a computer system including: an application server for processing the task requested from a client; and a customization executing server for changing an application provided by the application server. The customization executing server is configured to: manage user status information including operation information of the application; record the operation information of the application in the user status information based on a request message transmitted from the client; change a response message transmitted by the application server based on the user status information to newly generate a response message; and transmit the newly-generated response message to the client.