Abstract: It is intended to provide a regeneration promoter for regenerating a tissue with the use of somatic stem cells. It is also intended to provide a cell fusion promoter safely usable in vivo. Namely, it is intended to provide a cell fusion promoter comprising ATP or its metabolite. A cell fusion promoter comprising ATP or its metabolite and a method of producing fused cells in the presence of ATP or its metabolite. A medicinal composition for regenerating or improving the function of a tissue or an organ, which suffers from dysfunction or hypofunction due to injury or denaturation, by using stem cells. This composition comprises ATP or its metabolite and a pharmaceutically acceptable carrier.