[Problem] To provide a breast pump capable of securely holding a breast of a user and intensively stimulating a particular potion of the breast. [Means for Resolution] A breast pump 100 including a milk container main body 110; deformable means 170 for forming a sealed space; a horn member 150; space internal pressure altering means 190 for alternately providing a negative pressure condition and an atmospheric pressure condition in the sealed space; and a communicating portion 130, wherein: the horn member has a base end portion 151 disposed near the communicating portion and an opening end 152 disposed near an entrance through which the breast is inserted; the deformable means covers the inner surface of the horn member 150 and has a base end side attachable and detachable portion 171 to be fixed to the base end of the horn member and an opening side attachable and detachable portion 172 to be fixed to the opening end of the horn member; the deformable means has a stimulating convex 173 disposed between the base end side attachable and detachable portion and the opening side attachable and detachable portion; and the horn member has atmospheric pressure condition creating means for maintaining an atmospheric pressure condition in a space between the stimulating convex and the horn member.