9. An Fault Tree (FT) diagram generation aid method, comprising: an import procedure for obtaining a connection relationship of ruled lines and character strings from first data which is data of an FT diagram expressing a tree structure by the ruled lines and the character strings on a sheet of a spreadsheet program, acquiring an event included in the FT diagram and a connection relationship between events from an obtained connection relationship of the ruled lines and the character strings, and generating second data describing the tree structure of the FT diagram in a markup language based on the event included in the FT diagram and the connection relationship between events; and an editing procedure for editing the second data to generate third data describing the tree structure of the edited FT diagram in the markup language, wherein the import procedure generates the second data by: repeatedly executing first processing of setting a specific cell surrounded by a ruled line as a cell to be analyzed, discovering a lower event of the cell to be analyzed by following a ruled line extending on a right side of the cell to be analyzed and by searching a cell surrounded by a ruled line beyond the same, arranging an element of the lower event between a start tag and an end tag of the cell to be analyzed in the second data, and setting a cell of the lower event as a new cell to be analyzed until no additional new lower event is discovered after a cell of a top event is set as a first cell to be analyzed; repeatedly executing second processing of, when no additional new lower event is discovered, discovering a same-rank event of the cell to be analyzed by following a ruled line branching downward from a ruled line extending to a left side from the cell to be analyzed and by searching a cell surrounded by a ruled line beyond the same, arranging an element of the same-rank event in parallel with the element of the cell to be analyzed in the second data, setting the cell of the same-rank event as a new cell to be analyzed, and repeating the first processing until no additional new lower event is discovered, until no additional new same-rank event is discovered; and repeatedly executing third processing of, when no additional new same-rank event is discovered, setting a higher event of the same-rank event discovered immediately before as a new cell to be analyzed, and repeating the second processing until no additional new same-rank event is discovered, until the cell to be analyzed becomes the cell of the top event.
11. The FT diagram generation aid method according to claim 9 , further comprising: a consistency verification procedure for verifying the second or third data by obtaining a physical quantity of each event from the second or third data and by determining that consistency of the second or third data is not obtained if the physical quantity of a specific event and the physical quantity of a lower event thereof do not share the same unit and the physical quantity of the specific event cannot be expressed by multiplication, division or exponentiation of the unit of the physical quantity of the lower event.