1. An information searching method to be implemented in a computer, comprising: a selection accepting procedure accepting a first character string from characters or a character string in a reading or inspecting document that is displayed on a display unit; an index information acquiring procedure acquiring a second character string to be used as index information for searching the reading or inspecting document, based on identification information of the reading or inspecting document, and deleting the second character string that overlaps with the first character string from the index information; and a search control procedure executing a search using the first character string and the second character string as search keywords, wherein the index information acquiring procedure extracts each word having an occurrence that is greater than a predetermined value from the index information as the second character string, and wherein: the index information acquiring procedure extracts at least one of the words having a co-occurrence that is greater than the predetermined value with respect to the first character string from a plurality of second character strings based on a co-occurrence management table that manages the co-occurrence for each combination of words in the index information as a third character string, and the search control procedure executes the search based on the first character string and the third character string.