The present invention relates to a thermostable DNA polymerase-associated factor capable of enhancing DNA synthesizing-activity of a DNA polymerase; a thermostable DNA polymerase-associated factor possessing an activity of binding to a DNA polymerase and a method for producing the same; a gene encoding the DNA polymerase-associated factor; a method of DNA synthesis by using a DNA polymerase in the presence of the DNA polymerase-associated factor; and a kit comprising the DNA polymerase-associated factor. According to the present invention, there can be provided in vitro DNA synthesis and a DNA amplification system which are more excellent than conventional techniques by utilizing the DNA polymerase-associated factor of the present invention.