A thermostable DNA polymerase-related factor promoting the DNA synthesizing activities of DNA polymerases; a thermostable DNA polymerase-related factor having the activity of binding to DNA polymerases and a process for producing the same; genes encoding the above DNA polymerase-related factors; a method for synthesizing DNAs wherein DNA polymerases are used in the presence of the above DNA polymerase-related factors; and kits containing the above DNA polymerase-related factors. Use of these DNA polymerase-related factors makes it possible to provide DNA synthesis systems and DNA amplification systems in vitro which are superior to the conventional ones.