To effectively produce a gene transfer agent narrow in molecular weight distribution and excellent in gene transfer activity by a photo-living polymerization without an inexpensive photo-irradiation device.

The method for producing the gene transfer agent by a photo-living polymerization in a solution is characterized by making to exist a light-emitting compound which is decomposed in the solution to emit light having a peak wavelength of 360 to 375 nm. Since a reaction substrate can mainly be irradiated with main reaction light having a wavelength of nearly 370 nm causing the polymerization reaction (main reaction), without an expensive light-irradiation device which irradiates only a monochromatic light, a side reaction can be controlled to obtain the gene transfer agent comprising a branched polymer having a narrow molecular weight distribution.