The present invention provides an information processing method converting a format of a structured document, comprising: a first step copying, a first storage unit, information of a pre-converted first structured document sequentially by a predetermined amount beginning from the head; a second step adding start tags and/or end tags, and an identifier tag for identifying the aforementioned start and/or end tags so that the information copied to the first storage unit becomes one complete second structured document; a third step converting the second structured document built up in the first storage unit to a target format and outputting it to a second storage as a third structured document; and a fourth step removing the start tags and/or end tags and the identifier tag added to in the second step from the third structured document retained by the second storage unit and merging with a fourth structured document.