1. A method of testing a scenario which sequentially executes a plurality of application program interfaces (APIs), comprising; displaying a list of APIs each of which is a candidate to be executed next; receiving a selection of an API to be executed next from the list of APIs; receiving input data of the selected API; executing the selected API; storing sequence data indicating an execution sequence of the executed APIs; and storing association data indicating associations of input and output data between the executed APIs; wherein one or more candidate APIs are displayed based on the sequence data if the sequence data has already been stored in the step of displaying the list of APIs; and wherein the association data is displayed if the association data has already been stored in the step of receiving the input data of the selected API.