1. A computer-readable recording medium containing a form processing program for extracting prescribed keywords from a form image scanned, the form processing program causing a computer to perform a procedure comprising: recognizing a layout of the form image and extracting a readout region containing character images from the form image; recognizing characters from the character images of the extracted readout region through character recognition, and outputting the recognized characters as a character recognition result; extracting, from the character recognition result, characters which are included in strings of keywords each including an item and data thereof, the keywords being defined in form logical definitions according to logical structures which are common to forms of a same type; determining possible strings, including possible item strings for the items and possible data strings for the data of the items, by combining the extracted characters each of which satisfies positional relationships of one of the strings of the keywords; and linking the determined possible item strings and possible data strings according to positional relationships on the form image to determine combinations of the possible strings corresponding to the keywords; wherein: the form logical definitions include definitions of item strings representing the items, and when characters included in the item strings are extracted from the character recognition result, the determining casts the extracted characters for corresponding characters of the item strings and extracts as the possible item string a combination of the cast extracted characters which satisfies relationships of one of the item strings.