A plug which is detachably attached to a fluid port of a medical module, wherein a lid portion of the plug comprises a part to be penetrated through which a cylindrical tubular connection part of a fluid treatment device is capable of penetrating the lid portion, in a state where the plug is attached to the fluid port, when the connection part and the fluid port are connected so as to allow fluid to pass therethrough. The part to be penetrated has a thickness of 0.2 to 3.0 mm and is formed with a soft synthetic resin having a flexural elastic modulus of 100 to 1500 MPa or with an elastic material having a Shore A hardness of 40 to 90. Additionally, the plug comprises a connection part sealing part which liquid-tightly abuts the connection part in a state where the connection part is inserted inside the plug. Thus, the medical module and the fluid treatment device can be connected hygienically in a simple manner without removing the plug.
