Patent ID: 8167833

Claim:
A breast pump having a substantially conical milk expressing section configured to abut against a user's breast, and a breast pump main body which includes the milk expressing section and is attachable to and detachable from a bottle so as to communicate with the bottle, comprising: a pressure changing structure, connected to the milk expressing section, for alternately generating a negative pressure state and at least an atmospheric pressure state as a pressure higher than the negative pressure state; a deformable member configured to liquid-tightly separate a sealed space formed by abutment of the milk expressing section against the user's breast and a space connected to the sealed space, from the pressure changing structure, and which is capable of being deformed by transmission of the pressure that is changed by the pressure changing structure in order to adjust the pressure state of the milk expressing section; and a pressure adjustment structure for adjusting the negative pressure state by changing an amount of deformation of the deformable member while not adjusting a leakage volume of air.