A breast pump, including a housing container, a breast pump body having an attachment and detachment part, a negative pressure generating member attached to the breast pump body, and an operation unit. The breast pump body includes a diameter-expanded milking part, a small valve chamber, and an attachment part communicating with the small valve chamber and attaches thereto the negative pressure generating member that creates the negative pressure needed to express milk; the negative pressure generating member includes a connector to which the operation unit is connected, a comparatively soft attachment and detachment part having elasticity and attached detachably to the attachment part, a wall part having such rigidity that an external shape can be kept, and a deformable part provided on the inside of the wall part integrally with the wall part, is less thick than the wall part, and is deformed to create the negative pressure.