There is provided a multi-chamber bag that is capable of securely checking a medicinal substance accommodated therein without the necessity to perform a troublesome work, while preventing a matter deteriorating the medicinal substance from reaching the inside of a medicinal-substance accommodation chamber and hence securely preventing the deterioration of the medicinal substance. In a multi-chamber bag having a bag body that has a strong seal part that joins two sheet members together to define an interior space of the bag body, and a weak seal part that joints the two sheet members together so as to be able to rupture them apart, thereby partitioning the interior space of the bag body into a medicinal-substance accommodation chamber and a diluting-solution accommodation chamber, a pair of cover sheets are provided to respectively cover the medicinal-substance accommodation chamber. Each of the cover sheets is jointed to the facing sheet member so as to form an outside seal part surrounding the medicinal-substance accommodation chamber. One of the cover sheets has a structure capable of absorbing adverse influence causing matters, and a communication part for communication between spaces formed between both the sheet members and the both the cover sheets on both the sides is formed between an inside edge of the outside seal part and an inside edge of the strong seal part.
