A micro-bubble generating device is provided with a micro-bubble generating mechanism and a leading conduit provided with a widening section and a tube part, the widening section and the tube part in communication with each other in the leading conduit. The widening section has a hollow shape which has an axis Z as a central axis, and has base surfaces and a peripheral surface, and communicates with a nozzle of the micro-bubble-generating mechanism via one base surface of the widening section, and communicates with the tube part via the other base surface. The cross section orthogonal to a flow axis Z of the micro-bubbles of the widening section is larger than the cross section orthogonal to the flow axis Z of the tube part.