1. An Fault Tree (FT) diagram generation aid device, comprising: an import portion adapted to obtain 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, to acquire 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 to generate 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 portion adapted to edit the second data to generate third data describing the tree structure of the edited FT diagram in the markup language, wherein the import portion generates the second data by making a computer: repeatedly execute 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 execute 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 execute 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.