A plug for medical use includes a plug body having a column member made of a soft resilient material, and a core made of a material harder than the soft material of the plug body and disposed in the column member. The core displaces a portion of the column member outwardly and forming an outwardly bulging portion on the column member. The plug is inserted in a female opening of a BCP port of an extracorporeal blood circulation apparatus, for example, to prevent blood from leading from the BCP port.