The aim of the present invention is to provide a method of acquiring immunological tolerance to a foreign DNA or its expression product whereby the foreign DNA such as a vector carrying a foreign gene incorporated thereinto or its expression product can be recognized not as non-self but as self; a method of sustaining a gene therapeutic effect whereby a rejection to a foreign DNA such as a vector carrying a foreign gene incorporated thereinto or its expression product can be avoided; and a non-human animal which has acquired immunological tolerance to a foreign DNA such as a vector carrying a foreign gene incorporated thereinto or its expression product. Fetal immature T lymphocytes transferred with a foreign DNA, such as a foreign gene-incorporated viral vector, are introduced into thymus and said foreign DNA is expressed in the thymus organ. The methods of transferring said foreign DNA into a fetal immature T lymphocyte include, for example, co-cultivating the fetal immature T lymphocytes with viral vector-infected virus producer cells.