1. A failure cause estimation device comprising: a database that manages causes of past failure cases, questions, and replies thereto for each of the past failure cases; a condition presentation unit that presents combinations of the questions and the replies thereto managed in the database on a display screen as branch conditions used to classify a cause of a failure case; a condition selection unit that selects, according to a selection operation, at least one branch condition from the branch conditions presented by the condition presentation unit; a cause estimation unit that estimates, from contents of the database, at least one cause satisfying the at least one branch condition selected by the condition selection unit; and a cause presentation unit that presents the at least one cause estimated by the cause estimation unit on the display screen as at least one cause candidate.