PURPOSE: To enable a dart launching system to safely and surely launch darts in a sub-sea cementing head or sub-sea tool for oil well at prescribed timing by providing rotatable cooks and annulus sections surrounding the darts and launching down the darts with the pressure of a fluid by rotating the cocks.

CONSTITUTION: A dart launching system is constituted of a lower step section 1, a middle-step module section 2, and an upper-step swivel joint 3 and a side passage 4 is provided in the joint 3. Then rotatable cocks 7 and 7, darts 12 and 12, and annulus section 16 and 18 surrounding the darts 12 are provided so that the darts 12 may be launched down with the pressure of a fluid by rotating the cocks 7. When a pumped-up fluid is introduced to the side passage 4 and flows through the system, the first- and second-stage darts 12 are successively launched. Therefore, no heavy large manifold is required and, in addition, the quality of cementing work can be improved.