1. A document processing apparatus comprising: a first reading part that reads a form; a marking detection part that detects a marking written on the form from data read by the first reading part; an attribute name extraction part that extracts a character string described beforehand within or near a marking area of the detected marking as an attribute name; an attribute information memory that stores the attribute name extracted by the attribute name extraction part; a second reading part that reads a form on which attribute values are entered; an attribute name detection part that detects the attribute name stored in the attribute information memory and specifies a descriptive position of the detected attribute name from data read by the second reading part; and an attribute value extraction part that extracts a character string around the detection position of the attribute name detected by the attribute name detection part from the data read by the second reading part, and registers the extracted character string as the attribute value of an attribute associated with an attribute name in the attribute information memory, the attribute value extraction part selects the most valid character string as the attribute value by performing a selective natural language process for each character string included in each character string area and the attribute name, if the character string matched with the attribute name does not exist near the position on the read form corresponding to the descriptive position of the attribute name, the detection area of the attribute name is expanded from the area of the descriptive position of the attribute name over a selective region of the read form.