Abstract: The present invention provides a means for suppressing aggregation produced after purification of viral vector. That is, the present invention provides 1) a preparation method of viral vector including at least (1) a step for purification of viral vector and (2) a step of sterilization by filtration in the presence of serum albumin of the purified viral vector obtained in the aforementioned (1); 2) a pharmaceutical composition containing viral vector and serum albumin; 3) a method for suppressing aggregation of viral vector or a method for stabilizing viral vector, which includes adding serum albumin to a viral vector-containing solution; and 4) a pharmaceutical composition containing viral vector sterilized by filtration, which is substantially free of aggregation of viral vector and/or substantially free of degradation of virus activity upon preservation in a solution state at room temperature for at least 7 days.