The present disclosed subject matter provides a breast pump in which, by means of an operation performed by a user, the negative pressure (suctioning pressure) can be adjusted in accordance with the amount of the operation in a linear fashion without the occurrence of a steep gradient, and the negative pressure can be readily adjusted even during the expression of milk. The breast pump comprises: pressure changing means or structure, connected to a 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 which is provided so as to liquid-tightly separate a sealed space formed by abutment of the milk expressing section against the user's breast during the expression of milk, or a space connected to the sealed space, from the pressure changing means or structure, and which is capable of deforming elastically in order to adjust the negative pressure state of the milk expressing section by transmission of the pressure that is changed by the pressure changing means or structure; and pressure adjustment means or structure for adjusting the negative pressure by adjusting the amount of deformation of the deformable member.