To sufficiently enhance the transcription of the objective gene by joining strong promoter to a gene capable of coding an intranuclear factor of a human lymphoid cell and then transfecting the resultant genetic construct in a multi-copy state into a cell containing the objective gene.

A strong promoter is joined to a DNA capable of coding a structural gene of a regulation factor selected from the group consisting of four kinds of intranuclear factors IgNF-A, IgNF-E, IgNF-B and IgNF-κ 3, isolated from a nuclear extract of a lymphoid cell and capable of joining to a transcriptionally regulating DNA component of an Ig gene to prepare an expressible genetic construct. Furthermore, the resultant genetic construct in a multi- copy state so as to sufficiently enhance the transcription of the objective gene is then transfected into a cell containing the objective gene. The regulation factor can readily be identified by a high-sensitivity get electrophoretic DNA- binding assay for adding an alternating copolymer double-stranded poly(dI- dC).poly(dI-DC) thereto.