A novel means for safely and efficiently introducing a target substance such as nucleic acid, protein, or the like into cells (excluding immune cells) is provided by the present invention. Specifically, a system for delivering a target substance into a cell (excluding immune cells), including ultrafine bubble water or ultrafine bubble aqueous solution containing ultrafine bubbles with an average diameter of not more than 200 nm and not containing phospholipid, and an ultrasound generator in combination; a method for increasing the delivery of a nucleic acid, a protein or a low-molecular-weight compound into a cell (excluding immune cells) by using the ultrafine bubble water, etc.; and the like are provided.