A breast pump can be configured to be capable of easily attaching/detaching a primary side serving as a sealed space which is in communication with a milking space and allows the passage of breast milk, with a secondary side in which a case is connected with a pressure changing apparatus. The breast pump can include a breast pump main body connected to the pressure changing apparatus by a conduit, wherein a milking part is disposed so as to liquid-tightly separate a sealed space (or a space that is in fluid communication with the sealed space), and the pressure changing apparatus from each other. A pressure transmission part for transmitting pressure changed by the pressure changing apparatus can be provided. The pressure transmission part can include a deformable part where a volume in the sealed space can be deformed by a pressure fluctuation generated by the pressure changing apparatus. A case can accommodate the deformable part and be connected with the pressure changing apparatus at one end, and the other end of the case can include attachment structure for communicably attaching/detaching the case with the portion of the milking part that forms the sealed space.