A medical fluid bag (10) is formed of weak film material, and its inner cavity is partitioned into a plurality of compartments (20, 22) by a weak seal (18), with a discharge port (12) being disposed so as to open to one compartment (22). A halved cylinder-like medicine discharge prevention tool (28) is installed to cover the discharge port (12), and when put together it forms an inner cavity (34)complementary to the shape of the discharge port (12), with the discharge port (12) received in the inner cavity (34). The medicine discharge prevention tool (28) is put together under elasticity by acanti levered extension (38). The spreading and opening of the medicine bag caused by impactive fluid force produced in the medicine bag upon the opening of the weak seal opens flaps (30, 32) againstelastic force, thus opening the medicine discharge prevention tool (28). The medicine discharge prevention tool (28) more reliably eliminates the possibility of mal-operation of medicinal administration when the medicinal fluid bag (10) of multi-fluid mixing type remains unopened.