To provide a towable open tank capable of reliably detecting and displaying a full-tank state of a fluid to be transported and, as a result, avoiding a risk of occurrence of a secondary disaster.

The towable open tank 10 has a buoyance tube 11 formed into an oval shape when viewed from the plane thereof and a leakage preventing cover 12 provided on the buoyance tube 11. The circumference of a bag material 13 is mounted to the entire periphery of a lower portion of the buoyance tube 11 in a sealed state and, when the open tank 10 is used, the fluid 30 to be transported is put into a hull mainly composed of the bag material 13 and is transported by a tugboat under the condition that the buoyance tube 11 is filled with gas. The towable open tank 10 has a full-tank detecting means 17 provided at the mounting level of the leakage preventing cover 12 for detecting tension of stringlike objects 16 whose lower ends are fixed to the bag material 13 and for detecting a full-tank state of the fluid 30 to be transported, with which the bag material 13 is filled.