In a blood transfusion system (a blood transfusion kit, a blood transfusion kit for emergency blood transfusion, or a method of using a blood transfusion kit), a flow path through which blood flows is formed using a tube having a channel therein. The flow path includes: a first path that connects an upstream path and a downstream path and has a leukocyte removal filter removing leukocytes at an intermediate position of the channel and a second path that connects the upstream path and the downstream path and bypasses the leukocyte removal filter.
